Add a CONTRIBUTING.md

[DeFuex]

As the title says, create a contributing.md file for the sake of this repository. The rules written in this file should tell a user how coding conventions and general guidelines can be used to contribute to this repository.

[sn99]

@DeFuex try making the contributing.md by yourself if you have a desired end goal, the game should be made according to the principle that you have in mind