Add message about labeler.yml on release branch

Since the action makes an unqualified request to get the file contents
from the source repository, the file must exist on the default branch.

This note should remove some confusion people have had while 
performing initial setup.

README.md

@@ -10,6 +10,8 @@ GitHub Action allowing for applying labels to issues and pull requests based on
 
 This action requires a configuration file defined at `.github/labeler.yml` in your repository. The contents must follow either the *simple* schema or the *full* schema shown below.
 
+**NOTE** The file must exist on your release branch (e.g. `master`, `main`, etc.).
+
 Feel free to use one of the following schema examples to get started.
 
 #### Simple Schema