[AIRFLOW-XXXX] Add config to welcome first time contributors

[kaxil]

I have added a feature to boring-cyborg to greet new contributors when they open new PR and when their first PR gets merged.

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Link to JIRA issue: https://issues.apache.org/jira/browse/AIRFLOW-XXXX

• Description above provides context of the change
• Commit message starts with [AIRFLOW-NNNN], where AIRFLOW-NNNN = JIRA ID*
• Unit tests coverage for changes (not needed for documentation changes)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

(*) For document-only changes, no JIRA issue is needed. Commit message starts [AIRFLOW-XXXX].

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In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[potiuk]

awesome !