docs: Entities and Keys

[ryanseys]

In the docs, Entities and Keys section is marked TODO.

Is this because the feature is missing or because the feature is "there" but it's just undocumented? This clarification would help me understand the state of the project better.

[stephenplusplus]

This will go away when we merge our doc changes, which thankfully will simplify the readme. #127

[rakyll]

Is this because the feature is missing or because the feature

Oh, no. It's because we change interfaces 5 times a week :)

[ryanseys]

Yay! Awaiting magically simple README.