Add contribution guidelines

[cyrus01337]

More info can be found here.

Contributing is initially wonky and dark, given that there's a lot of custom functionality it becomes non-trivial to simply create a test command due to the complexity. As there will be more contributions made moving forward, ideally setting up some guidelines and providing concise breakdowns on how to achieve simple tasks would be beneficial. Such tasks/details may consist of the following:

• Initial setup (incl. Python version)
• Creating commands/cogs (simpler is better)
• Config description

I know that the project is currently undergoing a rewrite, once that is finalised it would be wise to pick this up then.