[milestone] Add MILESTONE file

This file is newly introduced into integration.git, so we should mirror
it here. It will soon be required for proper generation of release
version numbers. Set the milestone number to 0 on this test branch to be
consistent with the current tag.

Bug: 70302
Change-Id: I90c9788bd53a23d65d78c7e8566572af66655947
diff --git a/MILESTONE b/MILESTONE
new file mode 100644
index 0000000..573541a
--- /dev/null
+++ b/MILESTONE
@@ -0,0 +1 @@
+0