s/Warn/Explicitly note/ in the issue title :)

I think this is actually both important enough and subtle enough to warrant a ".. note::" callout in the docs.

In Python 3, it would say to use `sys.meta_path.insert(0, finder)` if you want your finder to take precedence over the default search locations, and `sys.meta_path.append(finder)` if you only want it handle cases that aren't already handled by the default machine.

In Python 2, it would say that custom finders on `sys.meta_path` will always take precedence over the default machinery, and if you want to do otherwise, you'll need to use `importlib2` and install its loaders into `sys.meta_path` ahead of your own.