Update readme with credit

After cleaning up my stackoverflow, I found a question that became
ClearAllButMatches. Once again it was Peter Rincker helping me create a
plugin.

Dug around to figure out who else helped contribute.

I don't know if I actually read scrooloose's post -- my initial visual
star search is from my first commit of vimfiles on Aug 2008. It grew for
a bit as a single command and eventually I caved and made it a function
to support more functionality.

The Ubuntu Brainstorm is dead, but this archived link works for now.