[Documentation] Improve installation docs

[rambip]

This repository does not contain a working installation guide. Usually, a repo like this contains 2 guides: one to easily deploy (with docker) and one to develop (for contributors)
There are multiple related problems in this repo that would be easy to fix:

1. The README mentions a DOCKER.md file to deploy using docker, which is not present.
2. We can at least refer to the official Decidim installation instructions for developpement: https://docs.decidim.org/en/v0.30/develop/guide_development_app
3. there are some major differences with the main Decidim app. In particular, there is no development_app rake task. The user should use bundle exec rake db:create db:migrate assets:precompile db:seed
4. If there are any additional dependencies, they should be specified.