Update README

[bgetting]

Looks great, @RickFrom1987!

We'll want to make sure that the README is updated to explain the following:

1. The normal usage of the original jquery-ajax-retry.
2. How to use the exponential retry, including the new options and what they do, as well as how to craft the retryCallback function and what arguments it receives.
3. Usage examples of both of the above use cases.