llvm-build: Add initial code for --write-make-fragment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143661 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/CommandGuide/llvm-build.pod b/docs/CommandGuide/llvm-build.pod
index 2ccc1a2..5ce93a9 100644
--- a/docs/CommandGuide/llvm-build.pod
+++ b/docs/CommandGuide/llvm-build.pod
@@ -48,6 +48,13 @@
 Write out new I<LLVMBuild.txt> files based on the loaded components. This is
 useful for auto-upgrading the schema of the files.
 
+=item B<--write-make-fragment>
+
+Write out the LLVMBuild in the form of a Makefile fragment, so it can easily be
+consumed by a Make based build system. The exact contents and format of this
+file are closely tied to how LLVMBuild is integrated with the Makefiles, see
+LLVM's Makefile.rules.
+
 =item B<--llvmbuild-source-root>=I<PATH>
 
 If given, expect the I<LLVMBuild> files for the project to be rooted at the