docs: Add HowToSetUpLLVMStyleRTTI.rst.

This document describes how to set up LLVM-style RTTI for a class
hierarchy. Surprisingly, this was not previously documented.

Also, link it into ProgrammersManual.html.

llvm-svn: 165293
diff --git a/llvm/docs/programming.rst b/llvm/docs/programming.rst
index 5d000cb..fe2604f 100644
--- a/llvm/docs/programming.rst
+++ b/llvm/docs/programming.rst
@@ -9,6 +9,7 @@
    CodingStandards
    CommandLine
    Atomics
+   HowToSetUpLLVMStyleRTTI
 
 * `LLVM Language Reference Manual <LangRef.html>`_
 
@@ -33,6 +34,11 @@
   Details the LLVM coding standards and provides useful information on writing
   efficient C++ code.
 
+* :doc:`HowToSetUpLLVMStyleRTTI`
+
+  How to make ``isa<>``, ``dyn_cast<>``, etc. available for clients of your
+  class hierarchy.
+
 * `Extending LLVM <ExtendingLLVM.html>`_
 
   Look here to see how to add instructions and intrinsics to LLVM.