[XRay] [docs] Document how to generate flamegraphs from xray traces.

Summary:
Updated the XRayExample docs with instructions for using the llvm-xray stacks
command.

Reviewers: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39106

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316192 91177308-0d34-0410-b5e6-96231b3b80d8