[AIRFLOW-6446] Add Github Action to Welcome First time contributors (#7031)

* [AIRFLOW-6446] Add Github actions to Welcome First time contributors

* Update the welcome message

Author: kaxil

.github/workflows/greetings.yml

@@ -0,0 +1,35 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+---
+
+name: Greetings
+on: [pull_request, issues]
+jobs:
+  greeting:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/first-interaction@v1
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          pr-message: |
+            Congratulations on your first Pull Request and welcome to the Apache Airflow community!
+            If you have any issues or are unsure about any anything please check our
+            Contribution Guide (https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst)
+
+            In case of doubts contact the developers at:
+            Mailing List: [email protected]
+            Slack: https://apache-airflow-slack.herokuapp.com/