README: Add `git submodule update --init` step to developer setup

This is a no-op if the user cloned with --recursive, but may be
required if not.

Change-Id: Iad1027908cb20f0e7ce92845582a966667683420

README

diff --git a/README b/README
index 4b07e4b..1514b2b 100644
--- a/README
+++ b/README
@@ -17,6 +17,7 @@
   ant
   sudo ln -s ${PWD}/bin/buck /usr/bin/buck
   cd /path/to/gitiles
+  git submodule update --init
   buck build all