CONTRIBUTING.md: use git tag -a for release tags (#6143)

git describe by default use annotated tags.

Rationale:

  Annotated tags, however, are stored as full objects in the Git
  database. They’re checksummed; contain the tagger name, email,
  and date; have a tagging message; and can be signed and verified
  with GNU Privacy Guard (GPG). It’s generally recommended that you
  create annotated tags so you can have all this information; but
  if you want a temporary tag or for some reason don’t want to keep
  the other information, lightweight tags are available too.

From: https://git-scm.com/book/en/v2/Git-Basics-Tagging#tagging

CONTRIBUTING.md

@@ -222,6 +222,7 @@ only pull requests targeted directly against master.
 - After simulation has successfully completed, create the release branch
   (`release/vX.XX.X`) from the `RC` branch
 - Create a PR to `master` to incorporate the `CHANGELOG.md` updates
+- Tag the release (use `git tag -a`) and create a release in Github
 - Delete the `RC` branches
 
 ### Point Release Procedure
@@ -240,7 +241,7 @@ releases will be based off of that release.
 - after tests and simulation have successfully completed, create the release branch `release/vX.XX.X` from the `RC` branch
 - delete the `pre-rc/vX.X.X` and `RC` branches
 - create a PR into `master` containing ONLY the CHANGELOG.md updates
-- tag and release `release/vX.XX.X`
+- tag (use `git tag -a`) then push the `release/vX.XX.X` branch & tag (`git push --tags`)
 
 ## Code Owner Membership