Reorganize documentation & update Readme

docs/Collection-of-elements.md

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+# Meritoo Common Library
+Common and useful classes, methods, exceptions etc.
+
+# Collection of elements
+
+Located here: `Meritoo\Common\Collection\Collection`. It's a set of some elements, e.g. objects. It's iterable and countable. Provides very useful methods. Some of them:
+- `getFirst()` - returns the first element in the collection
+- `getLast()` - returns the last element in the collection
+- `isEmpty()` - returns information if collection is empty
+- `add($element, $index = null)` - adds given element (at the end of collection)
+- `addMultiple($elements, $useIndexes = false)` - adds given elements (at the end of collection)
+- `prepend($element)` - prepends given element (adds given element at the beginning of collection)
+- `remove($element)` - removes given element
+
+Examples of usage below.
+
+### An empty collection
+
+```php
+use Meritoo\Common\Collection\Collection;
+
+$emptyCollection = new Collection();
+var_dump($emptyCollection->isEmpty()); // bool(true)
+```
+
+### Simple collection
+
+```php
+use Meritoo\Common\Collection\Collection;
+
+$elements = [
+    'lorem',
+    'ipsum',
+    123 => 'dolor',
+    345 => 'sit',
+];
+
+$simpleCollection = new Collection($elements);
+var_dump($simpleCollection->has('dolor')); // bool(true)
+```
+
+# More
+
+1. [Base test case (with common methods and data providers)](Base-test-case.md)
+2. [**Collection of elements**](Collection-of-elements.md)
+3. [Static methods](Static-methods.md)
+
+[‹ Back to `Readme`](../README.md)