Add guidance around not using mixins
Add guidance around not using mixins

Mixins will hopefully be removed from React eventually. In the meantime,
we can avoid the damage they cause by not using them. Most of this was
borrowed from @gaearon's blog post "Mixins Considered Harmful".

https://facebook.github.io/react/blog/2016/07/13/mixins-considered-harmful.html

xalexec/javascript