Enable auto-merge and describe it in CONTRIBUTING

Author: ralfhandl

.github/workflows/sync-dev-to-vX.Y-dev.yaml

@@ -61,11 +61,11 @@ jobs:
               --title "$BASE: sync with $HEAD" \
               --body "Merge relevant changes from \`$HEAD\` into \`$BASE\`.")
             echo ""
-            echo "PR to sync $DEV_BRANCH: $PR"
+            echo "PR to sync $BASE: $PR"
             sleep 10 # allow status checks to be triggered
             
             gh pr checks $PR --watch --required || continue
-            # gh pr merge $PR --merge --admin
+            gh pr merge $PR --merge --admin
           done
         env:
           GH_TOKEN: ${{ steps.generate-token.outputs.token }}

CONTRIBUTING.md

@@ -503,10 +503,9 @@ gitGraph TB:
 
 To keep changes in sync, we have some GitHub actions that open pull requests to take changes from `main` onto the `dev` branch, and from `dev` to each active version branch.
 
-- `sync-main-to-dev` opens a pull request with all the changes from the `main` branch that aren't yet included on `dev`.
-- `sync-dev-to-vX.Y-dev` opens pull requests with all the changes from `dev` that aren't yet included on the corresponding `vX.Y-dev` branch.
+- `sync-main-to-dev` opens a pull request with all the changes from the `main` branch that aren't yet included on `dev`. This pull request needs a single approval from either maintainers or TSC and can be merged.
+- `sync-dev-to-vX.Y-dev` opens pull requests with all the changes from `dev` that aren't yet included on the corresponding `vX.Y-dev` branch. These pull requests are automatically merged if all required status checks pass.
 
-These need a single approval from either maintainers or TSC and can be merged.
 The aim is to bring build script and repository documentation changes to the other branches.
 Published versions of the specifications and schemas will also move across branches with this approach.