Adds Github Action to Coveralls

Adds a brief example to the coveralls section for Github Actions.

Signed-off-by: Troy Fontaine <troy.fontaine@mogo.ca>

README.md

@@ -149,7 +149,7 @@ ignore:
   - ProjectTestsGroup/*
 ```
 
-And then in your `.travis.yml` or `circle.yml`, call `slather` after a successful build:
+And then in your `.travis.yml` or `circle.yml` or `github-action.yml`, call `slather` after a successful build:
 
 ```yml
 # .travis.yml
@@ -168,6 +168,20 @@ test:
 
 ```
 
+```yml
+# github-action.yml
+  myjob:
+    steps:
+      - run: |
+          bundle config path vendor/bundle
+          bundle install --without=documentation --jobs 4 --retry 3
+
+      - run: bundle exec slather
+        env:
+          GIT_BRANCH: ${{ github.event.pull_request.head.ref }}
+
+```
+
 #### Usage with Travis CI Pro
 
 To use Coveralls with Travis CI Pro (for private repos), add following lines along with other settings to `.slather.yml`: