Document how to build a LLVM pass with CMake out of source.

Patch by arrowdodger!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@129381 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/WritingAnLLVMPass.html b/docs/WritingAnLLVMPass.html
index 6d18d19..7e15245 100644
--- a/docs/WritingAnLLVMPass.html
+++ b/docs/WritingAnLLVMPass.html
@@ -211,6 +211,9 @@
 If your operating system uses a suffix other than .so (such as windows or 
 Mac OS/X), the appropriate extension will be used.</p>
 
+<p>If you are used CMake to build LLVM, see
+<a href="CMake.html#passdev">Developing an LLVM pass with CMake</a>.</p>
+
 <p>Now that we have the build scripts set up, we just need to write the code for
 the pass itself.</p>