docs: Fix Sphinx warning over Atomics.rst.

Atomics.rst was not linked into the toctree.

Docs now build warning-free!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164356 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/programming.rst b/docs/programming.rst
index 27e4301..5d000cb 100644
--- a/docs/programming.rst
+++ b/docs/programming.rst
@@ -8,12 +8,17 @@
 
    CodingStandards
    CommandLine
+   Atomics
 
 * `LLVM Language Reference Manual <LangRef.html>`_
 
   Defines the LLVM intermediate representation and the assembly form of the
   different nodes.
 
+* :ref:`atomics`
+
+  Information about LLVM's concurrency model.
+
 * `The LLVM Programmers Manual <ProgrammersManual.html>`_
 
   Introduction to the general layout of the LLVM sourcebase, important classes