add README for release instruction

distro/README.md

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+# Releasing rules_java
+
+1. Update version in distro/BUILD.bazel,
+2. Build the release running `bazel build //distro:rules_platform-{version}`
+3. Prepare release notes running `bazel build //distro:relnotes`
+4. Create a new release on GitHub
+5. Copy/paste the produced `relnotes.txt` into the notes. Adjust as needed.
+6. Upload the produced tar.gz file as an artifact.