Document eventuals code conventions

[CodingCanuck]

There are several code conventions and custom design patterns that are used throughout eventuals code, e.g.:

• All eventuals define a k() function, where k stands for kontinuation (aka continuation)
• Eventuals are usually implemented by a struct or class called _Foo that's an implementation detail, and users call a function called Foo

Let's document these patterns and conventions somewhere, perhaps in our contributing guide.