Feature/beginners warning

Summary:
Issue fixed: Instagram#811

I don't think this change needs a item in `CHANGELOG.md` as it's not a real bug fix or enhancement that influences the behaviour of the library.

- [X] All tests pass. Demo project builds and runs.
- [ ] I added tests, an experiment, or detailed why my change isn't tested.
- [X] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [X] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes Instagram#812

Differential Revision: D5283118

Pulled By: rnystrom

fbshipit-source-id: 518d38c7f620cef3e85cae5fd4a1c48567bcc48c

Source/IGListAdapter.m

@@ -272,6 +272,7 @@ - (void)performUpdatesAnimated:(BOOL)animated completion:(IGListUpdaterCompletio
     id<IGListAdapterDataSource> dataSource = self.dataSource;
     UICollectionView *collectionView = self.collectionView;
     if (dataSource == nil || collectionView == nil) {
+        IGLKLog(@"Warning: Your call to performUpdatesAnimated-method is ignored as dataSource or collectionView haven't been set.");        
         if (completion) {
             completion(NO);
         }
@@ -308,6 +309,7 @@ - (void)reloadDataWithCompletion:(nullable IGListUpdaterCompletion)completion {
     id<IGListAdapterDataSource> dataSource = self.dataSource;
     UICollectionView *collectionView = self.collectionView;
     if (dataSource == nil || collectionView == nil) {
+        IGLKLog(@"Warning: Your call to reloadDataWithCompletion-method is ignored as dataSource or collectionView haven't been set.");        
         if (completion) {
             completion(NO);
         }