RFA: document GDB's overlay support

[Andrew Cagney]

> That would be simpler.
> 
> I guess I wanted to provide instructions for putting your build system
> back in a "working normal" state: you type "cvs update", "configure",
> and "make", and things just work, without needing to remember to copy
> over certain files.
> 
> But obviously, the instructions should mention the simpler
> alternative, for those less forgetful than me.

It needs to work for the snapshot process - PDF, PS and HTML.  It 
definitly appears to do that!

v-nice description,
Andrew