.gitignore: Add *.mk, created as part of ChromiumOS build

Contributed by Josh Triplett <josh.triplett@intel.com>

BUG=None
TEST=Build Chromium and ChromiumOS from source, and run "repo status",
     with and without this change.

Review URL: https://webrtc-codereview.appspot.com/1000006

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@3334 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..9c8d698
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+# This file is for projects that checkout webrtc/ directly (e.g. Chromium).
+*.mk