Add instructions for updating major tag during release (GoogleCloudPl…

…atform#2409)

* Add instructions for updating major tag during release

* Update README.md (15 to 14)

* Unindent to match other code snippets in README.md

* Update docs/releasing/README.md

docs/releasing/README.md

@@ -64,4 +64,16 @@ This document walks through the process of creating a new release of Online Bout
 
 12. Make sure [cymbal-shops.retail.cymbal.dev](https://cymbal-shops.retail.cymbal.dev) works.
 
-13. [Publish your draft release on GitHub](https://github.com/GoogleCloudPlatform/microservices-demo/releases).
+13. Update the relevant major tag (for example, `v1`).
+
+  ```
+  export MAJOR_TAG=v0 # Edit this as needed (to v1/v2/v3/etc)
+  git checkout release/${TAG}
+  git pull
+  git push --delete origin ${MAJOR_TAG} # Delete the remote tag (if it exists)
+  git tag --delete ${MAJOR_TAG} # Delete the local tag (if it exists)
+  git tag -a ${MAJOR_TAG} -m "Updating ${MAJOR_TAG} to its most recent release: ${TAG}"
+  git push origin ${MAJOR_TAG} # Push the new tag to origin
+  ```
+
+14. [Publish your draft release on GitHub](https://github.com/GoogleCloudPlatform/microservices-demo/releases).