Document the build process for contributors

[glihm]

Currently, there's no explicit guide for a new contributor to what are the steps and options to build dojo and work on the code base.

This issue aims at adding a section into the README to explain new contributors how to:

1. Get dojo cloned
2. How to build (full workspace, how to work on a crate only, etc...)
3. How to test (same, full workspace, only some crates, etc...)
4. Which checks have to be done
5. use the scripts in scripts/