Document confirming navigation with ES6 classes

[taion]

Closes #2364

Here's one way to let users easily use Lifecycle when using ES6 classes. Zero LoC change on our end!

I think this is a bit cleaner than implementing on our wrapper - it'd essentially just be duplicating code from a perfectly good library that's already there.

[taion]

cc @brigand re: https://github.com/brigand/react-mixin#aside-do-i-need-mixins

Per #2364 (comment) I think the cleanest approach with ES7 is to apply this as a @Lifecycle decorator, but that's pretty much the same thing as just using the mixin.

Personally I don't think it's necessary or helpful for every library to really just re-implement this logic from react-mixin for creating mixin-style decorators, and anyway logically it is just a mixin, just with slightly different syntax.

I don't think that caveat in the react-mixin docs is necessarily warranted.

[filod]

@taion I lost this binding in routerWillLeave when using react-mixin

should we really use this?

[knowbody]

I'm not a big fan of react-mixin to be honest, not entirely sure about this solution.

@taion maybe we should rethink that?

[taion]

This is the abstraction breaking a bit because ES6 classes don't auto-bind.

I'm not hugely attached to react-mixin, but I can't really think of a better solution. We could make our own decorator, but it'd essentially be the same as just calling reactMixin.decorate(Lifecycle) code-wise, so I'm not sure if that would improve things.