Update readme (#19)

Fixes firebase deploy line for instances with only one environment not named "prod". Also added instructions for adding a custom environment name to that deploy command.

README.md

@@ -56,11 +56,14 @@ jobs:
       - name: Deploy to Firebase
         uses: w9jds/firebase-action@master
         with:
-          args: deploy --only hosting:prod
+          args: deploy --only hosting
         env:
           FIREBASE_TOKEN: ${{ secrets.FIREBASE_TOKEN }}
 ```
 
+If you have multiple hosting environments you can specify which one in the args line. 
+e.g. `args: deploy --only hosting:[environment name]`
+
 ## License
 
 The Dockerfile and associated scripts and documentation in this project are released under the [MIT License](LICENSE).