Add CONTRIBUTING

[mrombout]

• New examples must use libgba provided const, macros, functions, etc as much as possible to create a consistent set of examples.
• New examples must contain a representative screenshot of the example using the standard template.
• New examples should only introduce one concept. Unrelated concepts are permitted if they are required to fully demonstrate the original concept (e.g. Sprite effects (MAIN CONCEPT) example may use sprite rendering (CONCEPT 2)).