Add presubmit hooks for repository.

For now, I'll just add the presubmit hooks for the cos_patch_trailer.

Change-Id: Iaef8777b5a4ace508888c1a2d8e14f5f2c68a187
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
new file mode 100644
index 0000000..de7d854
--- /dev/null
+++ b/PRESUBMIT.cfg
@@ -0,0 +1,2 @@
+[Hook Overrides Options]
+cos_patch_trailer_check: true