Document helped-by and friends #543

The average dev has probably run across:

• Signed-off-by

And in fact, the initial welcome message has:

• Finally, the commit messages should end in a Signed-off-by: line matching the commits' author.

A dev who spends a lot of time in GitHub may run across:

• Co-authored-by

But, git appears to also have:

• Helped-by
• Reported-by
• Mentored-by

The bot doesn't seem to suggest anything else. And the repository doesn't have any documentation that mentions helped-by.