GitHub actions (#544)

* Create label.yml

* Create labeler.yml

* Create greetings.yml

.github/workflows/greetings.yml

@@ -0,0 +1,12 @@
+name: Greetings
+
+on: [pull_request]
+
+jobs:
+  greeting:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/first-interaction@v1
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        pr-message: 'Thank you for contributing to Apollo 11!'' first pr'