README: suggest rsrc tool for manifest embedding

Hi,

I've created a pure-Go tool named 'rsrc', intending it for use especially with WALK, for embedding the Windows manifest files properly as resources into Go programs - please take a look at: https://github.com/akavel/rsrc

I thought you might be interested in it, and possibly might like to hint at it as an easier option in the WALK docs.

You might also remember me as the guy who helped make callbacks from Windows to Go possible, also for WALK, one day in distant past, pre-Go1.

Cheers
/Mateusz Czapliński.