Init Testing Chapter

[4meta5]

This PR makes a few significant changes. At this point, it is ready for review.

CHANGELOG

It adds a CHANGELOG.md at the root. This file is intended to serve as a CHANGELOG similar to libp2p's changelog. It might track the growth of the recipes and reorganization of content.

In the long-run, it provides a transparent record of how things have changed without requiring that much buy in by the contributors; there should be no mandatory enforcement. When enough work has been done to warrant some bullets, this is better than nothing.

Testing Section

The new testing section is split up into three sections:

1. basic mock runtime
2. common test patterns
3. custom mock runtimes

The best way to review might be to checkout the branch and mdbook build --open. I'm eager to merge this, but am also open to review. I rewrote a lot of the code while testing, which makes a lot of sense. I started writing about the execution-schedule example in greater depth here but I don't want to include it in this PR. I'll bring it into a future one.

other

Instead of rust for code block annotations, I changed it to rust, ignore so that it doesn't give users the option to try to compile (which we know doesn't work for substrate code). I actually consider this to be a pretty huge win for UX (DX).

[JoshOrndorff]

what do you think about moving Instantiable Modules to section 6.3 (under Types and Traits) and adding a new section, section 8 for Testing with 8.1: Mock Runtime for Unit Testing?

Sounds good!

[JoshOrndorff]

Looks good.