Update readme with identifier syntax when overriding subclass methods

[halgorithm]

Ran into this issue today and I had to look through the closed issues to find the fix for it. It would be helpful if the readme mentioned it so others don't have to too!

[matthewrudy]

yeah, the identifier thing does work, but I haven't personally used it in a real application.

There are some edge cases, that won't work (memoize_all for example) but I don't know if anyone uses those.

If you give it a go, and everything is fine, then by all means issue a pull request with some extra documentation.