llvm dev - Jan 2019 - RFC: XRay Framework Documentation

Hi llvm-dev,

I’ve taken some time to document how one would implement an XRay mode that isn’t
one of the currently shipped modes with compiler-rt. The patch is at:

https://reviews.llvm.org/D56244

The documentation aims to show a couple of things:

- How to use the XRay APIs to programmatically start/stop tracing with a
specified mode.

- How to implement a new mode which registers itself with the XRay framework.

I’d really appreciate another pair of eyes on the wording and content. It would
be great to get feedback from those less familiar with some of the details of
the XRay runtime bits, and see whether the documentation covers enough
information to allow someone new to implement their own modes.

Thanks in advance!

Cheers

-- Dean