Add an internal notice file for internal checkouts.

Change-Id: Ibb298427efaab764d7ac32d3921b6ae3252c4a24
diff --git a/.INTERNAL_NOTICE.md b/.INTERNAL_NOTICE.md
new file mode 100644
index 0000000..467e71c
--- /dev/null
+++ b/.INTERNAL_NOTICE.md
@@ -0,0 +1,3 @@
+This file is used to warn developers of internal code checkouts. If you are
+reading this, don't be worried -- external contributors only see the OSS
+repositories, and you're safe.