Added USAGE doc

Author: BenEmdon

USAGE.md

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+# Usage
+
+How to install and use this module.
+
+1. [Installation](#installation)
+1. [Storyboard Usage](#storyboard-usage)
+1. [Programmatic Usage](#programmatic-usage)
+1. [Scroll to Edge Effect](#scroll-to-edge-effect)
+
+
+## Installation
+
+CenteredCollectionView is available through [CocoaPods](http://cocoapods.org) and [Carthage](https://github.com/Carthage/Carthage).
+
+To install it with **Cocoapods**, add the following line to your `Podfile`:
+```ruby
+pod "CenteredCollectionView"
+```
+
+To install it with **Carthage**, add the following line to your `Cartfile`:
+```
+github "BenEmdon/CenteredCollectionView"
+```
+
+## Storyboard Usage
+1. To start off lets add a `UICollectionView` to our controller, and lay it out however you want it.
+
+  ![AddCollectionView](/.github/AddCollectionView.gif)
+
+1. Next lets set the `UICollectionView`'s custom layout to be `CenteredCollectionViewFlowLayout`.
+
+  ![GiveFlowLayout](/.github/GiveFlowLayout.gif)
+
+1. Next we can optionally layout the prototype item. **Note that this doesn't determine the item's size.**
+
+  ![MakeItemBig](/.github/MakeItemBig.gif)
+
+1. Make sure the collection view cell reuse identifier is set.
+
+  ![CellIdentifier](/.github/CellIdentifier.png)
+
+1. Create an `IBOutlet` for the collection view.
+
+  ![IBOutlet](/.github/IBOutlet.gif)
+
+1. Next lets dive in to the code use:
+
+  ```swift
+  class ViewController: UIViewController {
+
+  	@IBOutlet weak var collectionView: UICollectionView!
+
+  	// The width of each cell with respect to the screen.
+  	// Can be a constant or a percentage.
+  	let cellPercentWidth: CGFloat = 0.7
+
+  	// A reference to the `CenteredCollectionViewFlowLayout`.
+  	// Must be aquired from the IBOutlet collectionView.
+  	var centeredCollectionViewFlowLayout: CenteredCollectionViewFlowLayout!
+
+  	override func viewDidLoad() {
+  		super.viewDidLoad()
+
+  		// Get the reference to the `CenteredCollectionViewFlowLayout` (REQURED STEP)
+  		centeredCollectionViewFlowLayout = collectionView.collectionViewLayout as! CenteredCollectionViewFlowLayout
+
+  		// Modify the collectionView's decelerationRate (REQURED STEP)
+  		collectionView.decelerationRate = UIScrollViewDecelerationRateFast
+
+  		// Assign delegate and data source
+  		collectionView.delegate = self
+  		collectionView.dataSource = self
+
+  		// Configure the required item size (REQURED STEP)
+  		centeredCollectionViewFlowLayout.itemSize = CGSize(
+  			width: view.bounds.width * cellPercentWidth,
+  			height: view.bounds.height * cellPercentWidth * cellPercentWidth
+  		)
+
+  		// Configure the optional inter item spacing (OPTIONAL STEP)
+  		centeredCollectionViewFlowLayout.minimumLineSpacing = 20
+
+  		// Get rid of scrolling indicators
+  		collectionView.showsVerticalScrollIndicator = false
+  		collectionView.showsHorizontalScrollIndicator = false
+  	}
+  }
+  ```
+
+## Programmatic Usage
+```Swift
+class ViewController: UIViewController {
+
+  let centeredCollectionViewFlowLayout = CenteredCollectionViewFlowLayout()
+  let collectionView: UICollectionView
+
+  override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?) {
+    // Initialize the collectonView with the `CenteredCollectionViewFlowLayout` (REQUIRED STEP)
+    collectionView = UICollectionView(centeredCollectionViewFlowLayout: centeredCollectionViewFlowLayout)
+    super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
+  }
+
+  override func viewDidLoad() {
+    super.viewDidLoad()
+
+    // Assign delegate and data source
+    collectionView.delegate = self
+    collectionView.dataSource = self
+
+    // Layout subviews (not shown)
+    ...
+
+    // Register collection cells (REQUIRED STEP)
+    collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: String(describing: UICollectionViewCell.self))
+
+    // Configure the required item size (REQURED STEP)
+    centeredCollectionViewFlowLayout.itemSize = CGSize(
+      width: view.bounds.width * cellPercentWidth,
+      height: view.bounds.height * cellPercentWidth * cellPercentWidth
+    )
+
+    // Configure the optional inter item spacing (OPTIONAL STEP)
+    centeredCollectionViewFlowLayout.minimumLineSpacing = 20
+
+    // Get rid of scrolling indicators
+    collectionView.showsVerticalScrollIndicator = false
+    collectionView.showsHorizontalScrollIndicator = false
+  }
+}
+
+// Delegate and datasource extensions
+...
+
+```
+
+## Scroll to Edge Effect
+![scrollToEdgeEnabled](/.github/ScrollToEdge.gif)
+Scrolling to an Edge on Touch 🎡
+
+Heres how you could trigger a scroll animation when a touch happens on an item that isn't the `currentCenteredPage`:
+
+```swift
+extension ViewController: UICollectionViewDelegate {
+  func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
+
+    // check if the currentCenteredPage is not the page that was touched
+    let currentCenteredPage = centeredCollectionViewFlowLayout.currentCenteredPage,
+    			currentCenteredPage != indexPath.row {
+      // trigger a scrollTo(index: animated:)
+      centeredCollectionView.scrollTo(index: indexPath.row, animated: true)
+    }
+  }
+}
+```