Update RELEASE.md with branch syncing instructions (#6471)

* Update RELEASE.md

* Update RELEASE.md

* Update RELEASE.md

* Update RELEASE.md

RELEASE.md

@@ -160,3 +160,41 @@ Add release to release page index under `/docs/pages/releases/index.mdx` with a
 Commit website update and open PR to add to `master`.
 
 Have PR reviewed and merged into `master`.
+
+## GitHub branch sync
+
+When we do a release we need to make sure `master` and `website_live` are both in sync with each other, this is done by:
+
+Creating a branch off `master` such as `bring-in-latest-website-changes` and merging in `website_live` changes, opening a PR and getting it merged, example - https://github.com/keystonejs/keystone/pull/6470
+
+Then the other way around, create a branch off `website_live` such as `bring-in-latest-master-changes`, and merging in `master` changes, opening a PR and getting it merged, example - https://github.com/keystonejs/keystone/pull/6472
+
+Once this is done, the histories will be out of sync, GitHub will state that `website_live` is still x commits behind, if you look at the branches page - https://github.com/keystonejs/keystone/branches
+
+To resolve this in the CLI:
+
+Go to the `website_live` branch
+
+`git checkout website_live`
+
+Check that we're identical to master
+
+`git diff master`
+
+Do a merge that should just update the parents of the new commit
+
+`git merge master`
+
+This should be empty
+
+`git diff origin/website_live`
+
+This should be empty
+
+`git diff master`
+
+Force push `website_live` (after turning off branch protection in github)
+
+`git push --force`
+
+Branches should now be nicely aligned.