[documentaiton] Add README.md with description of github actions ci infra.

PiperOrigin-RevId: 736943380

Author: toli-y

build_tools/ci/readme.md

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+This folder contains scripts and commands executed by GitHub actions in OpenXLA,
+TensorFlow and JAX OSS repos. The GitHub actions replaced
+KokoroPresubmit-tensorflow* workflows. They run under
+`Copybara_XLA{Presubmit,Submit}` presubmit chip (category) at Google internally.
+The tests run on several target OS configurations/GCP containers such as Linux
+x86 with GPU or Linux ARM64. They assure that XLA/Tensor Flow/JAX compiles and
+runs on those platforms. The tests uses released OSS c++ clang compiler which
+has some differences in supporting c++ standards compared Google's internal
+version.
+
+#### How it works
+
+Repo specific GitHub actions call `build.py --build="build_name"`. E.g. OpenXLA
+uses https://github.com/openxla/xla/blob/main/.github/workflows/ci.yml
+
+The build here is a set of shell script commands executing the test targets or
+doing compile only testing. Each GitHub action call translates into compile only
+test:
+
+1.  dryrun `bazel build --nobuild ... test_targets`
+1.  actual compile `bazel build ... test_targets`
+1.  analyse results `bazel analyze-profile ...`
+
+or compile and run test commands:
+
+1.  dry run `bazel build --nobuild ... test_targets`
+1.  actual test `bazel test ... test_targets`
+1.  analyse results `bazel analyze-profile profile.json.gz`
+
+Checking in changes to `build.py` regenerates `golden_commands.txt` which lets
+us see how commands are changing. `golden_commands.txt` are not called as part
+of the continuous integration process.
+
+#### When does it run?
+
+The GitHub actions are automatically called as part of Google presubmit. The
+GitHub actions are automatically called on GitHub PR commit if the committer is
+part of OpenXLA GitHub org. Committer which are not part of OpenXLA org need an
+approval from OpenXLA org member to run the actions.