Final mods for initial CONTRIBUTING.md

There are still a few sections that need to be written or expanded on.
>> This does not mean you have to write classes that are multithreaded

> Is this supposed to be "This does not mean you cannot write multithreaded code"?

No I was trying to say that people don't *have to* learn how to write parallel code (meaning they would have to put in locks, for example). They just have to write their classes so that objects of that type can peacefully coexist in different threads, meaning avoid globals. I'll clarify.