Print deprecation warning when local_manifest.xml is used

The preferred way to specify local manifests is to drop the file(s)
in the local_manifests folder.  Print a deprecation warning when
the legacy local_manifest.xml file is used.

Change-Id: Ice85bd06fb612d6fcceeaa0755efd130556c4464
diff --git a/docs/manifest-format.txt b/docs/manifest-format.txt
index c51b84b..9589352 100644
--- a/docs/manifest-format.txt
+++ b/docs/manifest-format.txt
@@ -250,4 +250,6 @@
 these extra projects.
 
 Additional remotes and projects may also be added through a local
-manifest, stored in `$TOP_DIR/.repo/local_manifest.xml`.
+manifest, stored in `$TOP_DIR/.repo/local_manifest.xml`. This method
+is deprecated in favor of using multiple manifest files as mentioned
+above.