[XRay][docs] Examples for how to use XRay

Summary:
This document is an attempt at showing how XRay could be used to debug
latency issues with LLVM tools, and how to use the llvm-xray tool to
analyse XRay traces.

Reviewers: echristo, mehdi_amini, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31493

llvm-svn: 299133