Update contribution guidelines with list of things for which we don't accept contributions

[cketti]

Every now and then we get pull requests for things we don't want to have changed by a contributor. A good deal of those can probably be avoided by updating our contribution guidelines.

Incomplete list of things I can think of right now:

• Fixing/implementing issues with the unconfirmed label
• Dependency updates
• Converting code to Kotlin

We should also include a short explanation on why we don't support contributions for these things.