Added CHANGELOG entry for #51

Summary:
Fixed #63. I used the style we use in CocoaPods, but I'm happy to adjust!

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/CONTRIBUTING.md)
Closes #67

Reviewed By: jessesquires

Differential Revision: D4015999

Pulled By: rnystrom

fbshipit-source-id: bbe8055f22e84c5bdc628b4c1d95dab111774e12

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 The changelog for `IGListKit`. Also see the [releases](https://github.com/instagram/IGListKit/releases) on GitHub.
 
+## Master
+
+- Fixed `-[IGListAdapter reloadDataWithCompletion:]` not returning early when `collectionView` or `dataSource` is nil and `completion` is nil. [Ben Asher](https://github.com/benasher44) [#51](https://github.com/Instagram/IGListKit/pull/51)
+
+
 1.0.0
 -----