oss-fuzz-gen: add README on how to build/test FI

scripts/oss-fuzz-gen-e2e/README.md

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+# OSS-Fuzz-gen helper scripts
+
+This foler contains various helper scripts to run [OSS-Fuzz-gen](https://github.com/google/oss-fuzz-gen).
+OSS-Fuzz-gen relies on Fuzz Introspector to provide data about the target under
+analysis, and relies on the web API to make the data available. The benefit
+of this is that the web API makes it possible to easily query data about
+many OSS-Fuzz projects ([https://introspector.oss-fuzz.com/api](https://introspector.oss-fuzz.com/api)).
+However, there are several locations where logic in Fuzz Introspector can be
+changed in order to impact OSS-Fuzz-gen:
+1) The frontends
+2) The post-processing
+3) The web api
+
+Each of these steps are part of the OSS-Fuzz-gen pipeline.
+
+The main purpose of the logic in this folder is to enable the full OSS-Fuzz-gen
+pipeline based on a local set up.
+
+
+## Usage
+
+Example running of this pipeline:
+
+```sh
+git clone https://github.com/ossf/fuzz-introspector
+cd fuzz-introspector/scripts/oss-fuzz-gen-e2e
+
+# Set up OSS-Fuzz-gen and build a version of OSS-Fuzz images based on the
+# current local Fuzz Introspector.
+./build_all.sh
+
+# Run an OSS-Fuzz-gen pipeline that relies on a local run of FI and a local
+# version of the webapp. This includes generating a new set of benchmarks for
+# the selected projects.
+export MODEL=gpt-3.5-turbo
+export OPENAI_API_KEY=...
+./run_all.sh cjson htslib
+
+# Show the web report created by OSS-Fuzz-gen
+cd workdir
+. .venv/bin/activate
+cd oss-fuzz-gen
+python3 -m report.web ./results/ 8012
+```
+
+The above steps can be rerun once changes in the FI has been applied. As such,
+these steps are the recommended steps to use during testing and development.