README: Add hint when using annotated tags

Until a proper fix is released,
I believe this will be helpful to to share this trick to community.

Bug: #290
Relate-to: #1506
Relate-to: #649
Origin: https://github.com/actions/checkout/pulls?q=author%3Arzr
Signed-off-by: Philippe Coval <philippe.coval@silabs.com>

README.md

@@ -26,6 +26,7 @@ Please refer to the [release page](https://github.com/actions/checkout/releases/
 
     # The branch, tag or SHA to checkout. When checking out the repository that
     # triggered a workflow, this defaults to the reference or SHA for that event.
+    # For tags, it can be forced to ${{ github.ref }} (useful if using git-describe)
     # Otherwise, uses the default branch.
     ref: ''