Create Best Practices and FAQ.md closes #405

Summary:
Initial best practices / FAQ. Closes  #405.
Closes #478

Differential Revision: D4568153

Pulled By: jessesquires

fbshipit-source-id: 57090288f66a79e3514c84ecb7199bd3d85890b0

Guides/Best Practices and FAQ.md

@@ -0,0 +1,67 @@
+# Best Practices and FAQs
+
+This guide provides notes and details on best practices in using `IGListKit`, general tips, and answers to FAQs.
+
+## Best Practices
+
+- We recommend adding an assert to check [`-isKindOfClass:`](https://developer.apple.com/reference/objectivec/1418956-nsobject/1418511-iskindofclass) on the object you receive in [`-didUpdateToObject:`](https://instagram.github.io/IGListKit/Protocols/IGListSectionType.html#/c:objc(pl)IGListSectionType(im)didUpdateToObject:) in your section controllers. 
+This makes it easy to track down easily-overlooked mistakes in your [`IGListAdapaterDataSource`](https://instagram.github.io/IGListKit/Protocols/IGListAdapterDataSource.html#/c:objc(pl)IGListAdapterDataSource(im)listAdapter:sectionControllerForObject:) implementation. 
+If this assert is ever hit, that means `IGListKit` has sent your section controller the incorrect type of object. 
+This would only happen if your objects provide *non-unique* diff identifiers. 
+
+```objective-c
+// Objective-C
+- (void)didUpdateToObject:(id)object {
+    NSParameterAssert([object isKindOfClass:[MyModelClass class]]);
+    _myModel = object;
+}
+```
+
+```swift
+// Swift
+func didUpdate(to object: Any) {
+    precondition(object is MyModelClass)
+    myModel = object as! MyModelClass
+}
+```
+
+- Make sure your [`-diffIdentifier`](https://instagram.github.io/IGListKit/Protocols/IGListDiffable.html#/c:objc(pl)IGListDiffable(im)diffIdentifier) implementation returns a **unique identifier** for each object.
+
+- We highly recommend using single-item sections when possible. That is, each section controller manages a single model (which may have one or multiple cells). This gives you the greatest amount of flexibility, modularity, and re-use for your components.
+
+## Frequently asked questions
+
+**How do you implement separators between cells?**
+
+See discussion in [#329](https://github.com/Instagram/IGListKit/issues/329)
+
+**How do I fix the error `Could not build Objective-C module 'IGListKit'`?**
+
+See discussion in [#316](https://github.com/Instagram/IGListKit/issues/316)
+
+**The documentation and examples have `<X>` feature or changes, but I don't have it in my version. Why?**
+
+This feature is on the `master` branch only and hasn't been officially tagged and [released](https://github.com/Instagram/IGListKit/releases). If you need to, you can [install from the `master` branch](https://instagram.github.io/IGListKit/installation.html).
+
+**Does `IGListKit` work with...?**
+
+- Core Data ([#460](https://github.com/Instagram/IGListKit/issues/460), [#461](https://github.com/Instagram/IGListKit/issues/461))
+- AsyncDisplayKit ([AsyncDisplayKit/#2942](https://github.com/facebook/AsyncDisplayKit/pull/2942))
+- ComponentKit ([ocrickard/IGListKit-ComponentKit](https://github.com/ocrickard/IGListKit-ComponentKit))
+- React Native
+- Reactive Cocoa
+- RxSwift
+
+Yes.
+
+**Does `IGListKit` work with `UITableView`?**
+
+No, but you can install the [diffing subspec via CocoaPods](https://instagram.github.io/IGListKit/installation.html).
+
+**What's the purpose of `IGListCollectionView`?**
+
+We use this subclass to gain compile-time safety to prevent disallowed methods from being called on `UICollectionView`, because `IGListKit` handles model and view updates. See discussion at [#409](https://github.com/Instagram/IGListKit/issues/409).
+
+**How can I manage cell selection and deselection?**
+
+See discussion at [#184](https://github.com/Instagram/IGListKit/issues/184).