Document release process (#532)

Signed-off-by: Lauri Nevala <lauri.nevala@gmail.com>

RELEASE_GUIDE.md

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+# Release Process
+
+Lens releases are built by CICD automatically on git tags. The typical release process flow is the following:
+
+1. Create a release branch `release/v{version}` from `master` branch or from existing release branch (for example, `release/v3.5`) on patch releases.
+2. Update changelog in `static/RELEASE_NOTES.md` and bump version in `package.json`.
+3. Create PR and put change log in description field.
+4. After the PR is accepted, create a tag from release branch.
+5. Push tag to GitHub.
+6. Publish automatically created GitHub release.
+7. Merge the release PR after the release is published and delete the release branch from GitHub.