README:provide detailed Windows instructions

[maxerbubba]

:)

[xirzec]

I appreciate the contribution, but I don't think the instructions using 'cp' is enough to warrant needing git bash. Maybe add a note that 'copy' should be used instead on Windows?

The reason I'm being a stickler is that it's important to me that folks understand WinJS doesn't require anything beyond Node/NPM to be installed in order to custom compile it, as some have complained in the core project about needing to install/be familiar with Git.