Instead of a single `CONTRIBUTING.md`, create an initial set of more
detailed documentation for the different types of contribution.
Start with `code.md` and `translate.md`, the processes for submitting
code and translations respectively.
Commit to documenting how to contribute documentation, project
management, user support, and social presence in the next week.
* Update CONTRIBUTING.md
Opened this file out of pure random curiosity and noticed a little bug. Since Tusky got three themes, this file was not up to date anymore.
* Update CONTRIBUTING.md
Removed outdated mention of API level 21 since anything below that isn't supported anymore.