Add contributing.md

[Dosplatos]

Hey there! This is a pretty awesome awesome-list, but it's lacking some guidelines for making contributions. I think that lead to the list becoming harder to navigate and read over time. I fixed the issues I found with alphabetization and missing anchor links to H2s that were missing in the table of contents in my latest PR.

Would it be helpful if a CONTRIBUTING.md was added to this repo to help mitigate those problems? I figured just basic stuff like requiring alphabetizing new additions and correctly adding new categories into the table of contents. I feel that would help keep this list that much more awesome!

Happy to help with this if that would be more convenient :)