Add bot comment with directions to update .mailmap

[epassaro]

Description

This new step should write a comment giving some directions to new contributors on how to add themselves to .mailmap.

However, this new feature can't be tested without merging since the pull_request_target trigger lives in the base of the pull request for security purposes.

The sample message is reproduced in a comment below.

Should we also mention the TARDIS Code of Conduct? @wkerzendorf

Motivation and context

Closes #1972

How has this been tested?

• Testing pipeline.
• Other.
  Not possible to test.

Examples

Type of change

• Bug fix.
• New feature.
• Breaking change.
• None of the above.

Checklist

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
  • (optional) I have built the documentation on my fork following the instructions.
• I have assigned and requested two reviewers for this pull request.

[codecov]

Codecov Report

Merging #1982 (82c1ec5) into master (01a018d) will not change coverage.
The diff coverage is n/a.

❗ Current head 82c1ec5 differs from pull request most recent head 68eaa57. Consider uploading reports for the commit 68eaa57 to get more accurate results

@@           Coverage Diff           @@
##           master    #1982   +/-   ##
=======================================
  Coverage   59.85%   59.85%           
=======================================
  Files          70       70           
  Lines        8080     8080           
=======================================
  Hits         4836     4836           
  Misses       3244     3244           

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[epassaro]

*beep* *bop*

Hi, human.

I'm the @tardis-bot and couldn't find your records in my database. I think we don't know each other, or you changed your credentials recently.

Please add your name and email to .mailmap in your current branch and push the changes to this pull request.

In case you need to map an existing alias, follow this example.