docs: Fix Sphinx warning over Atomics.rst.

Atomics.rst was not linked into the toctree.

Docs now build warning-free!

llvm-svn: 164356
diff --git a/llvm/docs/programming.rst b/llvm/docs/programming.rst
index 27e4301..5d000cb 100644
--- a/llvm/docs/programming.rst
+++ b/llvm/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