Add .github/workflows/self_hosted_runner_utils/README.md

This was meant to be part of jax-ml#13000, oops

Author: skye

.github/workflows/self_hosted_runner_utils/README.md

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+Configuration files for self-hosted Github Actions runners. We use self-hosted
+Cloud TPU VM runners for TPU CI.
+
+Googlers, see go/jax-self-hosted-runners for more information.