Expand and Verify Contributing Guide

The instructions in the contributing guide need to be verified for clarity and completeness - they should be tested and any additional information needed should be added.

Additionally, these changes should be made:

* Fix formatting: list formatting should be either bullets or numbers but not both
* 'To Contribute' section should be cleaned up and more detailed, the multiple parentheses make it confusing. This section should probably reference GitHub documentation on how to execute pushing/pulling/pull requests.
* There needs to be a reference to style guidelines as described in #77
* From #34: The instructions should be clear about which repository should be used to create a branch i.e. the main repository or the fork (Should probably be a branch from the contributor's fork)
* From #34: In step three the phrase pop-up box should be removed.

Inizia leggendo la guida per i contributi del repository e confrontala con le modifiche richieste in #77 e #34. Verifica la chiarezza e la completezza delle istruzioni, inclusi la selezione del branch, le indicazioni su GitHub push/pull e sulle pull request, le linee guida di stile, la formattazione e la rimozione di “pop-up box.”. Il lavoro è completo quando la guida riflette coerentemente questi requisiti.

Scritto dal modello di indicizzazione a partire dal testo della issue.