docs(repo): add RELEASE.md (#15750)

RELEASE.md

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+# Release workflow
+
+## Testnet release PRs and main development
+
+- All development work goes into the main branch.
+- When we release a testnet, we will open a release branch e.g., [alpha-6](https://github.com/taikoxyz/taiko-mono/tree/alpha-6).
+- For future releases we continue working on the main branch!
+- For bug fixes first try to fix on `main` and cherry-pick into the release branch. If not possible due to large changes on main, fix on the release branch and check if a similar bug fix is needed on main.