Fix documentation for find_with_capturable_params

In the README, find_with_capturable_params is erroneously shown to be
an instance method. Fix it to be a class method. Update a comment in
the Devise model to indicate that find_by_email is the default.
Merge pull request #12 from oreillymedia/fix-docs-on-overrides

Fix documentation for find_with_capturable_params