Add README

README.MD

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+# PHP Captcha Library
+## What is it
+A library implemented by PHP to generate captcha easily.
+## Requirements
+- PHP 5 +
+- PHP GD
+- composer
+> Please assure that you have installed php-gd, and install it via `apt install php7.0-gd` or `apt install php5-gd` is recommended.
+## Installation
+Here are two ways to install:
+- Using composer:
+    ```
+    composer require php-quickorm/captcha
+    ```
+
+- or download the `Captcha.php` and `arial.ttf` to your project and import it by `require "Captcha.php";`
+
+## Usage
+
+The captcha has three levels, and the first is the easiest.
+
+### Simple
+image.php
+```php
+$captcha = new Captcha();
+$_SESSION['code'] = $captcha->getCode();
+$captcha->render();
+```
+
+index.html
+```html
+<img src="image.php">
+```
+### Advanced Options
+
+```php
+// The Captcha has three levels, and the first is the easiest.
+$level = 3; 
+// Case Sensitive
+$caseSensitive = false;
+
+
+$captcha = new Captcha($level, $caseSensitive);
+
+// Get the code
+$code = $captcha->getCode();
+
+// Get the php-gd image resource
+$im = $captcha->getImageResource();
+
+// Check the code if correct
+$captcha->check("AbcD");
+
+// Send HTTP response as a image
+$captcha->render();
+```
+
+Here is a demo: https://github.com/php-quickorm/Captcha/tree/master/demo
+