add Greeting action (#843)

* add Greeting action

* Update greetings.yml

Co-authored-by: William Falcon <waf2107@columbia.edu>

.github/workflows/greetings.yml

@@ -0,0 +1,14 @@
+name: Greetings
+# https://github.com/marketplace/actions/first-interaction
+
+on: [pull_request, issues]
+
+jobs:
+  greeting:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/first-interaction@v1
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        issue-message: 'Hi''! thanks for your contribution!, great first issue!'
+        pr-message: 'Hey thanks for the input! please give us a bit of time to review it!'