Expand google-java-format-diff

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=235723967
diff --git a/scripts/google-java-format-diff.py b/scripts/google-java-format-diff.py
index e9e7133..958cd8b 100755
--- a/scripts/google-java-format-diff.py
+++ b/scripts/google-java-format-diff.py
@@ -20,6 +20,10 @@
   git diff -U0 HEAD^ | google-java-format-diff.py -p1 -i
   svn diff --diff-cmd=diff -x-U0 | google-java-format-diff.py -i
 
+For perforce users:
+
+  P4DIFF="git --no-pager diff --no-index" p4 diff | ./google-java-format-diff.py -i -p7
+
 """
 
 import argparse