Documentation for contributors

[Omar Muscatello]

We should add documentation to contribute to this project, including but not limited to:

• branching strategy
• naming conventions;
  • branches;
  • PR titles;
• how to run the project on a local machine;
• how to test components;
• which parts of the projects should be updated when submitting changes (e.g. changes to the .md files);
• how to handle components' behavior which can only be implemented with JS.

Before working on that, we should have a more exhaustive list.

[Francesco Improta]

I've added some basi information about contributing (creating an issue, using labels, etc).

We could open a separated issue for development details.