Add relationship docs.

Author: yajra

documentation.md

@@ -42,6 +42,7 @@
 	- [Query Builder Extension](/docs/{{package}}/{{version}}/query-builder)
 	- [Regex Search](/docs/{{package}}/{{version}}/regex)
 	- [Smart Search](/docs/{{package}}/{{version}}/smart-search)
+	- [Relationships](/docs/{{package}}/{{version}}/relationships)
 - Sorting/Ordering
 	- [Manual Order](/docs/{{package}}/{{version}}/manual-order)
 	- [Order Column](/docs/{{package}}/{{version}}/order-column)

relationships.md

@@ -0,0 +1,51 @@
+# Eager Loading Relationships
+
+`Datatables` support searching and sorting of eager loaded relationships when using `Eloquent`.
+In this example, I will show you how to setup a eager loading search using `EloquentEngine`.
+
+To enable search, we need to eager load the relationship we intend to use using Laravel's `User::with('posts')` api.
+
+```php
+use Datatables;
+
+Route::get('user-data', function() {
+	$model = App\User::with('posts');
+
+	return Datatables::eloquent($model)
+				->addColumn('posts', function (User $user) {
+                    return $user->posts->map(function($post) {
+                        return str_limit($post->title, 30, '...');
+                    })->implode('<br>');
+                })
+				->make(true);
+});
+```
+
+To trigger search on `posts` relationship, we need to specify the `relation.column_name` as the `name` attribute in our `javascript` appropriately.
+
+```php
+<script>
+	$(document).ready(function() {
+		$('#users-table').DataTable({
+        processing: true,
+        serverSide: true,
+        ajax: '{{ url("collection/basic-object-data") }}',
+        columns: [
+            {data: 'id', name: 'id'},
+            {data: 'name', name: 'name'},
+            {data: 'email', name: 'email'},
+            {data: 'posts', name: 'posts.title'},
+            {data: 'created_at', name: 'created_at'},
+            {data: 'updated_at', name: 'updated_at'}
+        ]
+	});
+</script>
+```
+
+Looking at `{data: 'posts', name: 'posts.title'},`:
+- `data: posts` represents the data key (`data.posts`) that we are going to display on our table.
+- `name: posts.title` represents the `User` model relationship (`posts`) and the column we are going to perform our search (`title`).
+
+## Nested Relationships
+Same strategy goes for nested relationships but do **NOTE** that ordering is not yet fully tested on nested relationships.
+