Add FAQ about line nums in Mocha browser test reporter

answer: atm my setup for Mocha's reporter doesn't read src maps, though, of course, the browser itself does. Can I find a setup that does work with Mocha?

This may be solvable within Mocha itself

Start from the issue text and investigate how Mocha's browser test reporter handles source maps and line numbers. Confirm whether a working setup exists within Mocha, then document the answer as an FAQ entry; done means the setup or limitation is clearly explained.

Written by the indexing model from the issue text.