README improvements

Various clarifications and reformulations based on my experience using
the README to build and run Gargantext. In particular:
- Information related to development has been given its own section
- Docker Compose has been added to the list of prerequisites
- Information pertaining to using Stack has been moved to an annex file
- The content to be copypasted into a `cabal.project.local` file has been moved
  out of the README into a separate file `cabal.project.local_toCopy`

Those may not be the best decisions, I am happy to discuss them if
needed