Add AppStream metainfo file

[malfisya]

From their website :

AppStream allows upstream projects to define metadata about the components they provide using small XML files, metainfo files, which get installed into locations on the client system and are used by distributors to enhance their metadata.

This small xml files are required to enable bzflag to be displayed in modern Linux system application store (e.g: Gnome Software and KDE Discover). Solus and Flathub is already carrying a patch to enable this. This file is installed to /usr/share/metainfo/. This is the AppStream metainfo we carried :

<?xml version="1.0" encoding="UTF-8"?>
<component type="desktop">
  <id>org.bzflag.BZFlag</id>
  <launchable type="desktop-id">org.bzflag.BZFlag.desktop</launchable>
  <project_license>LGPL-2.1 or MPL-2.0</project_license>
  <metadata_license>CC0-1.0</metadata_license>
  <name>BZFlag</name>
  <developer id="org.bzflag">
    <name>BZFlag Developers</name>
  </developer>
  <summary>A 3D first person tank battle game</summary>
  <description>
    <p>
    BZFlag is a 3D networked multi-player multi-platform tank battle game. It runs on Windows, macOS, Linux, BSD, and other platforms. It was one of the most popular games ever on Silicon Graphics machines and continues to be developed and improved to this day.
    </p>
  </description>
  <url type="homepage">https://www.bzflag.org</url>
  <url type="bugtracker">https://github.com/BZFlag-Dev/bzflag/issues</url>
  <url type="help">https://www.bzflag.org/help</url>
  <screenshots>
    <screenshot type="default">
      <image>https://www.bzflag.org/assets/images/media/screenshots/churchyard_03.jpg</image>
      <caption>A map called Churchyard by Louman</caption>
    </screenshot>
    <screenshot>
      <image>https://www.bzflag.org/assets/images/media/screenshots/japanese_castles_01.jpg</image>
      <caption>A map called Japanese Castles by krungthai</caption>
    </screenshot>
    <screenshot>
      <image>https://www.bzflag.org/assets/images/media/screenshots/missile_wars_2.3_01.jpg</image>
      <caption>A two team capture-the-flag map called Missile Wars 2.3 by Ducatiwannabe</caption>
    </screenshot>
  </screenshots>
  <releases>
    <release version="2.4.26" date="2022-11-20">
      <description>
        <p>This release mainly fixes a few client bugs:</p>
        <ul>
          <li>Audio pops should be fixed.</li>
          <li>Positional audio in observer mode now uses the camera direction.</li>
          <li>Fixed a regression that caused some textures to render skewed.</li>
          <li>Fixed the Machine Gun firing continuously when the mouse wheel is used to fire.</li>
        </ul>
      </description>
    </release>
    <release version="2.4.24" date="2022-03-08">
      <description>
        <p>This release works around an event polling regression in SDL 2.0.20 that caused our game to appear unresponsive. There's a few other small changes as well.</p>
      </description>
    </release>
    <release version="2.4.22" date="2021-02-27">
      <description>
        <p>This release fixes several issues with SDL 2 window handling. There were also some other minor fixes.</p>
      </description>
    </release>
    <release version="2.4.20" date="2020-04-24">
	  <description>
		<p>This release added several new functions/events to the bzfs API and reorganized the client menus to improve usability.  It also adjusts some default options, such as radar size, to improve the new user experience.</p>
	  </description>
	</release>
    <release version="2.4.18" date="2018-10-18">
      <description>
        <p>This release fixes audio on Windows.</p>
      </description>
    </release>
    <release version="2.4.16" date="2018-09-23">
      <description>
        <p>This release primarily fixes a bug introduced in 2.4.14 that corrupted replay file headers. This release reverts that change, and can also read the corrupted header for replays made with the bad code.</p>
      </description>
    </release>
    <release version="2.4.14" date="2018-05-03">
      <description>
        <p>This release had several fixes and some new features.</p>
        <ul>
        <li>The mute/kill/kick/ban messages no longer display admin callsigns, and the ban message shows the remaining ban time.</li>
        <li>World weapon API has been revamped, including new bz_eServerShotFiredEvent and bz_eAllowServerShotFiredEvent events.</li>
        <li>New showMotto permission to control which players are allowed to set a motto.</li>
        <li>Custom BZDB variables defined by plugins no longer require using -setforced to set them from configuration file.</li>
        <li>The number pad keys are now treated uniquely for key mapping with SDL2.</li>
        <li>Antialiasing now works when starting the client windowed.</li>
        </ul>
      </description>
    </release>
    <release version="2.4.12" date="2017-10-30">
      <description>
        <p>This release had a few additional features and some minor fixes.</p>
        <ul>
        <li>Multisampling (anti-aliasing) now works on all platforms with SDL</li>
        <li>The permission for running /poll set is no longer assigned by default</li>
        <li>The macOS build uses a newly released version of SDL that fixes an issue with retina macs when running in a scaled resolution</li>
        <li>It is possible to silence all unregistered players in your client by silencing - (the minus sign or hyphen), similar to how silencing * (asterisk) silences all players. Additionally, the /silence and /unsilence commands both work correctly now and support both special cases.</li>
        </ul>
      </description>
    </release>
    <release version="2.4.10" date="2017-03-12">
      <description>
        <p>This release had a few additional features and some minor fixes.</p>
        <ul>
        <li>Multisampling support under SDL 2 on macOS</li>
        <li>Added an option to invert one or both joystick axes</li>
        <li>Add ability to reload badwords list with '/reload badwords'</li>
        <li>It is possible for bzfs plugins to register new types of polls</li>
        <li>Fix high DPI handling on Windows (again, since it broke when we switched to using SDL2)</li>
        </ul>
      </description>
    </release>
  </releases>
  <content_rating type="oars-1.0">
    <content_attribute id="social-chat">intense</content_attribute>
  </content_rating>
</component>

It would be nice if this file is included in the release tarball and maintained by bzflag developer. Thank you for your attention. Have a good day. Cheers!