Improved the documentation

Summary:
Improved the documentation so it that the Getting Started guide calls out that you need to set `dataSource`-property of the `IGListAdapter` as I totally misse that.
Closes Instagram#820

Differential Revision: D5353295

Pulled By: rnystrom

fbshipit-source-id: deca1d75fad08f69081033ae1a65302a4f10ba9f

Guides/Getting Started.md

@@ -62,6 +62,12 @@ func emptyView(for listAdapter: ListAdapter) -> UIView? {
 }
 ```
 
+After you have created the data source you need to connect it to the `IGListAdapter` by setting its `dataSource` property:
+
+```swift
+adapter.dataSource = self
+```
+
 You can return an array of _any_ type of data, as long as it conforms to `IGListDiffable`.
 
 ### Immutability

Source/IGListAdapter.m

@@ -282,7 +282,7 @@ - (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.");        
+        IGLKLog(@"Warning: Your call to %s is ignored as dataSource or collectionView haven't been set.", __PRETTY_FUNCTION__);        
         if (completion) {
             completion(NO);
         }
@@ -319,7 +319,7 @@ - (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.");        
+        IGLKLog(@"Warning: Your call to %s is ignored as dataSource or collectionView haven't been set.", __PRETTY_FUNCTION__);
         if (completion) {
             completion(NO);
         }