Add note about global gitignore in our .gitignore files.
See discussions in I7ca6a7fca, I66f1fd9c1 and I9efffccfa
Change-Id: Ice44d016697b359d92ebaa5a1b8348b9d3afc59e
diff --git a/.gitignore b/.gitignore
index 19377ad..d3c9a94 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,6 @@
+# NOTE: we don't include IDE-specific files (e.g. IntelliJ's *.iml and .idea/) on-purpose.
+# Those patterns should go in a global gitignore or repository-specific excludes.
+# See http://www.gwtproject.org/makinggwtbetter.html#global_gitignore
+
# Working folder of ant build script
/build/