Make the SDK docs usable, up-to-date, and comprehensive.

[johnnymatthews]

The SDK docs are not good, and they're out of date. Let's revamp them so that they're usable, up-to-date, and comprehensive. This is a parent issue with lots of child issues springing off it. I'll edit this issue as we create more and more child issues.

• Project README (i.e. the README.md over at github.com/entropyxyz/sdk/): Improve the readability and this repo's README  sdk#443
• Introduction: Define the purpose of the SDK #288
• Get started: Tell users how to get started with the SDK. #289