feat: add greetings action

.github/workflows/greetings.yml

@@ -0,0 +1,13 @@
+name: Greetings
+
+on: [pull_request, issues]
+
+jobs:
+  greeting:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/first-interaction@v1
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        issue-message: 'Welcome to OpenRPC! Thank you for taking the time to create an issue. Please review the [guidelines](https://github.com/open-rpc/spec/blob/master/CONTRIBUTING.md)'
+        pr-message: 'Welcome to OpenRPC! Thank you for taking the time to create an issue. Please review the [guidelines](https://github.com/open-rpc/spec/blob/master/CONTRIBUTING.md)'