[docs] Guide prospective TableGen backend writers.

Boilerplate is often the hardest part of getting started with these
kinds of things, so throw them a bone.

llvm-svn: 173969
diff --git a/llvm/docs/TableGenFundamentals.rst b/llvm/docs/TableGenFundamentals.rst
index 526795f..4fe4bb9 100644
--- a/llvm/docs/TableGenFundamentals.rst
+++ b/llvm/docs/TableGenFundamentals.rst
@@ -791,6 +791,10 @@
 TableGen backends
 =================
 
+Until we get a step-by-step HowTo for writing TableGen backends, you can at
+least grab the boilerplate (build system, new files, etc.) from Clang's
+r173931.
+
 TODO: How they work, how to write one.  This section should not contain details
 about any particular backend, except maybe ``-print-enums`` as an example.  This
 should highlight the APIs in ``TableGen/Record.h``.