Add Development section to the README

[faultyserver]

The Installation section covers installation instructions for end users of Myst, but there's no documentation on how to set up the development environment to be able to contribute to Myst directly.

Since there are no dependencies other than Crystal itself, the setup is pretty minimal, but it should cover cloning the repo, getting the right version of Crystal, and running the tests at the very least.

Additional information would include a run through of the project structure but that may be getting too long for the README.