[XRay] Documentation up for Review

Hi Everyone,

TL;DR: See https://reviews.llvm.org/D26386 for initial documentation for XRay.

I've spent some time documenting how to use XRay from stuff that's already available in LLVM, Clang, and compiler-rt.

This is the first time I'm contributing to documentation for LLVM, and it'd be greatly appreciated if someone good at word-smithing would have a look. :slight_smile:

Cheers

-- Dean

As an update, this is now live at http://llvm.org/docs/XRay.html -- feedback would be most appreciated!

Cheers

-- Dean