Clearly state expectations in keys and values for {{}} and usermapping

[roberthopman]

First, thank you.

Second, wanted to make a note (not a PR), for future users:

• for the values in settings.ts, clearly state {{}} is redundant.
• correct way to use usermapping can use a note on a usecase: 'UserNameFromGitLab': 'UserNameFromGitHub'

[jslabontednd]

I just lost a few hours to this as well.