New guide for UICollectionView (follow-up to #409)

[jessesquires]

In light of #409, we should publish updates to the Guides, or maybe a new guide?

Proposal, new guide:

Working with UICollectionView

• details the history (legacy) of IGListCollectionView
• links to [Remove IGListCollectionView] Should IGListAdapter really depend on IGListCollectionView? #409
• lists the methods you should never call on UICollectionView when using ListKit

Thoughts?

[jessesquires]

cc @rnystrom

[rnystrom]

Also prefetchingEnabled = NO, using UIAppearance trick is cool too

[weyert]

Sorry, why is prefetchingEnabled = NO wasn't this one of the hot new features of iOS 10 announce last year at WWDC?

[rnystrom]

@weyert yup, it was. However we've run some testing and show that by simply being enabled this feature causes serious scroll performance regressions.

That is, if prefetchingEnabled = YES and don't even have a prefetchDataSource set, your app will scroll much worse.

IGListKit doesn't use this feature (we have our own "working range" implementation that is similar), so recommend always disabling it.

[weyert]

Ah, makes sense. Thanks for clarifying it, @rnystrom

[jessesquires]

PR up