add contribution document

Author: streamtw

docs/_layouts/default.html

@@ -37,6 +37,7 @@ <h2 class="project-tagline">{{ site.description | default: site.github.project_t
         <span><a href="customization">Customization</a></span>
         <span><a href="events">Events</a></span>
         <span><a href="upgrade">Upgrade</a></span>
+        <span><a href="contribution">Contribution</a></span>
       </nav>
     </section>
 

docs/contribution.md

@@ -0,0 +1,13 @@
+## Steps to contribute
+1. Fork [unisharp/laravel-filemanager](https://github.com/UniSharp/laravel-filemanager) from GitHub.
+1. Run commands below:
+
+    ```
+    git clone git@github.com:UniSharp/laravel-filemanager-example-5.3.git
+    cd laravel-filemanager-example-5.3
+    composer require unisharp/laravel-filemanager:dev-master
+    make init
+    ```
+1. Edit codes in `vendor/unisharp/laravel-filemanager`
+1. Push your changes to your fork.
+1. Send a pull request to [unisharp/laravel-filemanager](https://github.com/UniSharp/laravel-filemanager).