Adding a readme for the collections library

src/Collection/README.md

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+# CakePHP Collection Library
+
+The collection classes provide a set of tools to manipulate arrays or Traversable objects.
+If you have ever used underscore.js, you have an idea of what you can expect from the collection classes.
+
+## Usage
+
+Collections can be created using an array or Traversable object.  A simple use of a Collection would be:
+
+```php
+use Cake\Collection\Collection;
+
+$items = ['a' => 1, 'b' => 2, 'c' => 3];
+$collection = new Collection($items);
+
+// Create a new collection containing elements
+// with a value greater than one.
+$overOne = $collection->filter(function($value, $key, $iterator) {
+    return $value > 1;
+});
+```
+
+The `Collection\CollectionTrait` allows you to integrate collection-like features into any Traversable object
+you have in your application as well.
+
+## Documentation
+
+Please make sure you check the official documentation at http://book.cakephp.org/3.0/en/core-libraries/collections.html
+
+