Add contributing docs

[VaarunSinha]

We should add a contributing.md which could have these contents:

• Which packages we are using to build this bot
• Code of conduct
• General guidelines etc...

We could steal some contents from: https://www.pythondiscord.com/pages/guides/pydis-guides/contributing/contributing-guidelines/
Code of conduct: https://www.pythondiscord.com/pages/code-of-conduct/

[knyghty]

Ok, I added a simple CoC. I'll add contribution docs when I get time - that's a bit more complex.