Extend Indigo tutorial with information about full smart contract development

Clarification and motivation

Indigo tutorial describes language features that allow you to write contract code, but apparently it does not mention tools that let you and other people use your contract. In other words, how to go from just having Indigo code into a full project consisting Indigo contract(s). Specifically:

1. Lorentz.ContractRegistry which is the primary way to print a contract and related data.
2. Documentation generator.
3. Testing framework.

Acceptance criteria

Indigo tutorial should have some kind of "best practices" for smart contract development and mention relevant tools, namely:

1. Insert documentation into your contract (directly in code).
2. Use Lorentz.ContractRegistry to print your contract.
3. Write tests for your contract using our testing framework.

Maybe I've missed something else important btw.