Document what the convenient way of developing the project generator is

[nietaki]

(for the sake of contributors)

Worth documenting that you don't need to install the archive to see how it works, you can just use it from the root directory of the project and we usually use "demo" as the project name (mix raxx.new demo) as it's already in gitignore.

Also that in some fringe situations docker-compose up -- build helps when docker seems to be acting in a weird way.

[CrowdHailer]

Yep, this sounds a good idea.