Add sync to m help

Add sync to the list of targets documented by m help.

Change-Id: Id6cdb9d6e332b85068d895120aa7d344cf287f37
diff --git a/help.sh b/help.sh
index e51adc1..c405959 100755
--- a/help.sh
+++ b/help.sh
@@ -26,6 +26,8 @@
     clean                   (aka clobber) equivalent to rm -rf out/
     checkbuild              Build every module defined in the source tree
     droid                   Default target
+    sync                    Build everything in the default target except the images,
+                            for use with adb sync.
     nothing                 Do not build anything, just parse and validate the build structure
 
     java                    Build all the java code in the source tree