Add custom cell guide.

CUSTOM_CELLS.md

@@ -0,0 +1,76 @@
+# MessageKit Custom Cell Guide(s)
+
+- [How can I add a custom cell?](#how-can-i-add-a-custom-cell)
+
+## How can I add a custom cell?
+
+**Note:** If you choose to use the `.custom` kind you are responsible for all of the cell's layout. Any `UICollectionViewCell` can be returned for custom cells which means any of the styling you provide from the `MessageDisplayDelegate` will not effect your custom cell. Even if you subclass your cell from `MessageContentCell`.
+
+Creating a custom cell involves four parts. First, you will need to create your custom cell so that it inherits from `UICollectionViewCell`. Second, you will need to set the size of your custom cell by creating a class that inherits from `MessageSizeCalculator`. Third, you will need to add your custom message size calculator to a class you make that inherits from `MessagesCollectionViewFlowLayout`. Fourth, you will need to register your custom cell and reference your custom collection view flow layout.
+
+### Example
+
+Let's take a look at what it takes to create a custom cell that displays a red block for your custom message.
+
+- **1. Custom Cell**: We create a cell that inherits from `UICollectionViewCell`
+**MyCustomCell.swift**
+```
+open class MyCustomCell: UICollectionViewCell {
+    open func configure(with message: MessageType, at indexPath: IndexPath, and messagesCollectionView: MessagesCollectionView) {
+        self.contentView.backgroundColor = UIColor.red
+    }
+}
+```
+
+- **2. MessageSizeCalculator**: We set the size of our cell by creating a class that inherits from `MessageSizeCalculator`
+**CustomMessageSizeCalculator.swift**
+```
+open class CustomMessageSizeCalculator: MessageSizeCalculator {
+    open override func messageContainerSize(for message: MessageType) -> CGSize {
+    //HERE - Customize to size your content appropriately. This just returns a constant size.
+    return CGSize(width: 300, height: 130)
+    }
+}
+```
+
+- **3. MessageFlowLayout**: We add our custom message size calculator to our collection view layout by creating a class that inherits from `MessagesCollectionViewFlowLayout`
+**MyCustomMessagesFlowLayout.swift**
+```
+open class MyCustomMessagesFlowLayout: MessagesCollectionViewFlowLayout {
+    lazy open var customMessageSizeCalculator = CustomMessageSizeCalculator(layout: self)
+
+    override open func cellSizeCalculatorForItem(at indexPath: IndexPath) -> CellSizeCalculator {
+        let message = messagesDataSource.messageForItem(at: indexPath, in: messagesCollectionView)
+        if case .custom = message.kind {
+            return customMessageSizeCalculator
+        }
+        return super.cellSizeCalculatorForItem(at: indexPath);
+    }
+}
+```
+- **4. Implementation**: We register our custom cell and reference our newly created `MyCustomMessagesFlowLayout.swift`
+**ConversationViewController.swift**
+```
+internal class ConversationViewController: MessagesViewController {
+    override func viewDidLoad() {
+        super.viewDidLoad()
+        messagesCollectionView = MessagesCollectionView(frame: .zero, collectionViewLayout: MyCustomMessagesFlowLayout())
+        messagesCollectionView.register(MyCustomCell.self)
+        //...
+    }
+//...
+    override open func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
+        guard let messagesDataSource = messagesCollectionView.messagesDataSource else {
+            fatalError("Ouch. nil data source for messages")
+        }
+
+        let message = messagesDataSource.messageForItem(at: indexPath, in: messagesCollectionView)
+        if case .custom = message.kind {
+            let cell = messagesCollectionView.dequeueReusableCell(MyCustomCell.self, for: indexPath)
+            cell.configure(with: message, at: indexPath, and: messagesCollectionView)
+            return cell
+        }
+        return super.collectionView(collectionView, cellForItemAt: indexPath)
+    }
+}
+```