add callout to checkout main when updating major version tag

CONTRIBUTING.md

@@ -101,7 +101,7 @@ major version number (e.g. `v1`) in their workflows while
 automatically getting all the
 minor/patch updates.
 
-To do this just force-create a new annotated tag and push it:
+To do this just checkout `main`, force-create a new annotated tag, and push it:
 ```
 git tag -fa v2 -m "Updating v2 to 2.3.4"
 git push origin v2 --force