Added Subspec for Diffing

Summary:
So the main Podspec file now has two Subspecs, `Diffing` and for lack of better name `Default`. `Diffing` is purely the files within the common directories (used for Diffing, if in the future we support MacOS NSCollectionView then this may need changing). `Default` depends on `Diffing` but also adds all the non-common files.

To use it via CocoaPods, nothing changes. You `import IGListKit` and you will only have access to the files in the pod you use. (So if you use `IGListKit/Diffing` in your Podfile, then you will only get access to the diffing files). If you do a manual installation, or I assume via Carthage, then you will need to import the correct header file (either `IGListDiffKit.h` or `IGListKit.h` depending on what you want).

Turns out CocoaPods creates it's own umbrella header (unless you tell it otherwise) meaning our ones are unused by it.

Just to confirm `IGListKit` in your Podfile will give you everything, only if you add `/Diffing` will you "opt-out" o
Closes #368

Differential Revision: D4367438

Pulled By: jessesquires

fbshipit-source-id: 272318ca551e7e8c4177ca3ca501fde23bd9705a

CHANGELOG.md

@@ -17,6 +17,8 @@ This release closes the [2.1.0 milestone](https://github.com/Instagram/IGListKit
 
 - Working ranges now work with `IGListStackedSectionController`. [Ryan Nystrom](https://github.com/rnystrom) [(#356)](https://github.com/Instagram/IGListKit/pull/356)
 
+- Added CocoaPods subspec for diffing, `IGListKit/Diffing` and an [installation guide](https://instagram.github.io/IGListKit/installation.html). [Sherlouk](https://github.com/Sherlouk) [(#368)](https://github.com/Instagram/IGListKit/pull/368)
+
 ### Fixes
 
 - Avoid `UICollectionView` crashes when queueing a reload and insert/delete on the same item as well as reloading an item in a section that is animating. [Ryan Nystrom](https://github.com/rnystrom) [(#325)](https://github.com/Instagram/IGListKit/pull/325)

Examples/Examples-iOS/Podfile.lock

@@ -1,5 +1,9 @@
 PODS:
- - IGListKit (2.0.0)
+ - IGListKit (2.0.0):
+ - IGListKit/Default (= 2.0.0)
+ - IGListKit/Default (2.0.0):
+ - IGListKit/Diffing
+ - IGListKit/Diffing (2.0.0)
 
 DEPENDENCIES:
  - IGListKit (from `../../IGListKit.podspec`)
@@ -9,7 +13,7 @@ EXTERNAL SOURCES:
  :path: ../../IGListKit.podspec
 
 SPEC CHECKSUMS:
- IGListKit: b85413574889e5aa8958ab8f61ec634caf77f924
+ IGListKit: 9927a71a17642dddbb38856259d3b91ac7d415e3
 
 PODFILE CHECKSUM: 55363d0329e656c259d9b9982b162afd8d8904c6
 

Examples/Examples-macOS/Podfile.lock

@@ -1,5 +1,9 @@
 PODS:
- - IGListKit (2.0.0)
+ - IGListKit (2.0.0):
+ - IGListKit/Default (= 2.0.0)
+ - IGListKit/Default (2.0.0):
+ - IGListKit/Diffing
+ - IGListKit/Diffing (2.0.0)
 
 DEPENDENCIES:
  - IGListKit (from `../../IGListKit.podspec`)
@@ -9,7 +13,7 @@ EXTERNAL SOURCES:
  :path: ../../IGListKit.podspec
 
 SPEC CHECKSUMS:
- IGListKit: b85413574889e5aa8958ab8f61ec634caf77f924
+ IGListKit: 9927a71a17642dddbb38856259d3b91ac7d415e3
 
 PODFILE CHECKSUM: 1c6e95fa41b8dad60a4f701a82d85b3b49e979b8