docs: Sphinxify GoldPlugin document.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165198 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/LinkTimeOptimization.rst b/docs/LinkTimeOptimization.rst
index 53d673e..7eacf0b 100644
--- a/docs/LinkTimeOptimization.rst
+++ b/docs/LinkTimeOptimization.rst
@@ -29,6 +29,8 @@
 helps to do optimizations that are not possible in other models. The linker
 input allows the optimizer to avoid relying on conservative escape analysis.
 
+.. _libLTO-example:
+
 Example of link time optimization
 ---------------------------------