[Doc] CONTRIBUTING.md Guidelines

[dudash]

Is your asset request related to a problem? Please describe.
Yes, folks new to the project need some guidelines on how to contribute

Is this a new asset or a modification/increment to an existent asset?
New asset

Describe alternatives you've considered
None

Additional context
Typical of github projects to name this CONTRIBUTING.md
For example:
https://github.com/godotengine/godot/blob/master/CONTRIBUTING.md