chore(bigframes): document release procedure using legacylibrarian (#17436)

Fixes #<522923981> 🦕

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: shuoweil

packages/bigframes/release-procedure.md

@@ -0,0 +1,51 @@
+# BigQuery DataFrames (bigframes) release procedure
+
+*(Note: bigframes releases are marked with `skip_release: true` in `librarian.yaml` and must be kicked off manually using legacylibrarian.)*
+
+## Setup (First Time Only)
+
+*   Install `legacylibrarian`:
+
+        go install github.com/googleapis/librarian/cmd/legacylibrarian@latest
+
+*   Authenticate with GitHub CLI:
+
+        gh auth login
+
+## Release Steps
+
+*   Obtain GitHub token:
+
+        export LIBRARIAN_GITHUB_TOKEN=$(gh auth token)
+
+*   Stash changes (repo must be clean):
+
+        git stash -u
+
+*   Fetch and checkout base:
+
+        git fetch origin main
+        git fetch origin --tags
+        git checkout origin/main
+
+*   Check image updates:
+
+        legacylibrarian update-image --push
+
+*   Create release PR:
+
+        # Option A: Push directly
+        legacylibrarian release stage --repo=https://github.com/googleapis/google-cloud-python --library=bigframes --library-version=X.X.X --push
+
+        # Option B: Manual edit first (omit --push, edit files in /tmp/librarian-*, commit/push from there)
+        legacylibrarian release stage --repo=https://github.com/googleapis/google-cloud-python --library=bigframes --library-version=X.X.X
+        # In /tmp repository:
+        git commit -a -m "chore: create release" --no-verify  # keep librarian config pristine
+        git push origin HEAD
+        gh pr create --fill --label "release:pending"
+
+*   Post-release restore:
+
+        # Move back any stashed/relocated files (like .vscode)
+        git checkout main
+        git stash pop