Add API documentation

[Tyriar]

Consumers can only find the true capabilities of the generator by looking through examples or searching the interface. The bigger parts like syntax, terminal, background fallbacks etc. should be called out on the readme.

[rdnlsmith]

Hi, would you mind if I worked on this? I'm interested in using said capabilities, so I'll need to explore it anyways.

[Tyriar]

@rdnlsmith sounds great 😃

[Tyriar]

Also here is my theme for reference https://github.com/Tyriar/vscode-theme-sapphire/blob/master/index.ts