[XRay][Docs] Add documentation for XRay in LLVM

Summary:
This is the initial version of the documentation for how to use XRay as
it stands in LLVM, Clang, and compiler-rt. We leave some room for later
expansion mentioining what is work in progress and what could be
expected moving forward.

We also give a high level overview of future work that's both ongoing
and planned.

Reviewers: echristo, dblaikie, chandlerc

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D26386

llvm-svn: 286319
diff --git a/llvm/docs/index.rst b/llvm/docs/index.rst
index 29f2bd8..634e19a 100644
--- a/llvm/docs/index.rst
+++ b/llvm/docs/index.rst
@@ -273,6 +273,7 @@
    MIRLangRef
    Coroutines
    GlobalISel
+   XRay
 
 :doc:`WritingAnLLVMPass`
    Information on how to write LLVM transformations and analyses.
@@ -394,6 +395,9 @@
 :doc:`GlobalISel`
   This describes the prototype instruction selection replacement, GlobalISel.
 
+:doc:`XRay`
+  High-level documentation of how to use XRay in LLVM.
+
 Development Process Documentation
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