add installation.md

installation.md

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+## Requirements
+
+- PHP >= 5.3.0
+- [PHP cURL extension](http://php.net/manual/en/book.curl.php)
+- [PHP Mcrypt extension](http://php.net/manual/en/book.mcrypt.php){{fn(optional, required while enable the encrypt transfer feature)}}
+
+[Laravel 5 SDK wrapper](https://github.com/overtrue/laravel-wechat)
+
+## Installations
+1. By [composer](http://getcomposer.org/):
+
+  ```shell
+  composer require overtrue/wechat
+  ```
+
+2. By Downlads
+
+ - [download the lastest version zip package](https://github.com/overtrue/wechat/archive/master.zip)
+ - [or the archived version](https://github.com/overtrue/wechat/releases)
+
+ and then point-out the package `autoload.php` file such as:
+
+  ```php
+  <?php
+
+  require __DIR__ . "/wechat/autoload.php";
+  ...
+  ```