README rejig

[eadmundo]

I want to restructure and simplify the README. There's loads of good content in there, but I think it's more verbose than it needs to be, and we should structure it to focus on a more basic use case initially - the IETF draft standard is pretty complex compared to e.g. Shopify. We also should document the options themselves rather than only through the default methods.

The TOC was a bit deeply indented and intimidating, perhaps we don't need it.

I've started to structure it like this:

• Brief description
• Install
• Usage [by code example, really]
• Options [in more of an API-doc format]
• IETF draft standard [explanation of how this works - lots of the content can go here]
• Shopify?

I am wondering if before we go deeply into perfecting this README we should extract the IETF hmac methods into a external lib