Add an Explicit Example of Optional Env Var

Summary:
The convention from other actions is to take inputs with the `with:` object
which is not the case for this action. This change adds an explicit example
to the README to hint developers where the key/value is expected.

README.md

@@ -27,6 +27,7 @@ jobs:
       uses: anothrNick/github-tag-action@master
       env:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        WITH_V: true
 ```
 
 #### Options