docs: Fix Sphinx warning over yaml2obj.rst.

yaml2obj.rst was not included in the toctree

Input from Michael Spencer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164355 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/userguides.rst b/docs/userguides.rst
index fa6e3cf..c7197ef 100644
--- a/docs/userguides.rst
+++ b/docs/userguides.rst
@@ -14,6 +14,7 @@
    Lexicon
    Packaging
    HowToAddABuilder
+   yaml2obj
 
 * `The LLVM Getting Started Guide <GettingStarted.html>`_