Rd enhancement: Add file in which roxygen found the source comments

[tbates]

Roxy says things like this at the top of Rd files she generates

% Generated by roxygen2 (4.0.0.99): do not edit by hand

It would be helpful if this read

% Generated by roxygen2 (4.0.0.99): do not edit by hand
% instead, edit roxygen comments in file whereIcameFrom.R

I ask because in a big package it's hard to see where people have put the roxygen comments

[hadley]

I did want to do this, but unfortunately it's infeasible with the current design of roxygen2.

[krlmlr]

@hadley: #291 works for me so far, would you like to take a look?