Xray docs with description of Flight Data Recorder binary format.

Xray docs with description of Flight Data Recorder binary format.

Summary:
Adding a new restructuredText file to document the trace format produced with
an FDR mode handler and read by llvm-xray toolset.

Reviewers: dberris, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36041

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309836 91177308-0d34-0410-b5e6-96231b3b80d8