Better doc

[raphael-jolimoi]

The library is really hard to use. It is quite useful and powerful but the doc doesn't live up to basic usages of the library.

For example:

• There isn't a list of available methods
• There isn't a clear way explaining what the library expects from the API calls and what happens to the axios instance as a consequence
• It lacks examples

[ajmas]

I have tried documenting the provider options in #241. As for other parts, if anyone can contribute suitable documentation, then I am sure the developers would appreciate it.