add support for gist.github.com

README.md

@@ -40,6 +40,9 @@ Refer [here](https://github.com/actions/checkout/blob/v1/README.md) for previous
     # Default: ${{ github.repository }}
     repository: ''
 
+    # Gist name with owner. For example, schacon/1
+    gist: ''
+
     # 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.
     # Otherwise, defaults to `master`.