Refactor readme

[mkrd]

Let's face it, the readme could be a lot better. It should be refactored to conform to current conventions, provide a more complete overview of the library and to increase readability

[twodayslate]

Alternatively / in addition you could make use of the GitHub wiki feature or use Jazzy to automatically create your documentation (similarly to BigInt).

[twodayslate]

@mkrd can you enable docs as mentioned in #38?

[twodayslate]

Docs: http://mkrd.github.io/Swift-BigInt/

@mkrd if you could put this in the Website section for the project?

Then I think this can be closed after a ./generate-docs.sh

[twodayslate]

I can't for the life of me to get jazzy to work after getting a new mac. I think moving to swiftdoc (and the associated GitHub Action) is the best way forward.