Add description to README.md

Author: meyfa

README.md

@@ -1,5 +1,100 @@
-# phpunit-assert-gd
+# AssertGD for PHPUnit
 
 [![Build Status](https://travis-ci.org/meyfa/phpunit-assert-gd.svg?branch=master)](https://travis-ci.org/meyfa/phpunit-assert-gd)
 
-PHPUnit matcher/assertions for GD image resources
+Trying to assert images with PHPUnit? This project provides a constraint and the
+required assertions that allow you do to so.
+
+It supports comparing **files on disk** as well as **image resources** in
+memory.
+
+## Installation
+
+Add this package to your Composer dev-dependencies:
+
+```
+composer require --dev meyfa/phpunit-assert-gd
+```
+
+## Examples
+
+The assertions are available as a
+[trait](http://php.net/manual/en/language.oop5.traits.php), so you can easily
+`use` them in your test case class:
+
+```php
+<?php
+use AssertGD\GDAssertTrait;
+
+class ExampleTest extends PHPUnit_Framework_TestCase
+{
+    // this trait adds the assert methods to your test case
+    use GDAssertTrait;
+
+    public function testSomething()
+    {
+        $this->assertSimilarGD('./tests/expected.png', './tests/actual.png');
+    }
+}
+```
+
+**Note:** While this library should work with PHP down to at least v5.3.3,
+traits are a v5.4.0 feature. For versions lower than v5.4.0, you have to use
+this alternative syntax:
+
+```php
+<?php
+use AssertGD\GDSimilarityConstraint;
+
+class ExampleTest extends PHPUnit_Framework_TestCase
+{
+    public function testSomething()
+    {
+        $this->assertThat('./tests/actual.png',
+            new GDSimilarityConstraint('./tests/expected.png'));
+    }
+}
+```
+
+### Plain comparisons
+
+Use `assertSimilarGD` if you expect 2 images to be exactly equal.
+Use `assertNotSimilarGD` if you expect there to be differences.
+
+```php
+$this->assertSimilarGD('./tests/img.png', './tests/same.png');
+$this->assertNotSimilarGD('./tests/img.png', './tests/other.png');
+```
+
+### Threshold values
+
+Provide a number between 0 and 1 to set the error threshold. For example, a
+value of 0.2 would allow for at most 20% difference.
+
+```php
+$this->assertSimilarGD('./tests/img.png', './tests/similar.png', '', 0.2);
+```
+
+### Parameter types
+
+Instead of file paths, you can pass in GD image resources. This eliminates
+having to write something to disk prior to the comparison.
+
+```php
+$img = imagecreatetruecolor(10, 10);
+$this->assertSimilarGD('./tests/empty-10x10.png', $img);
+imagedestroy($img);
+```
+
+### Manual constraint
+
+If you need to configure mock objects or do other, more complex matching calls,
+use `isSimilarGD` to obtain a constraint object (similar to what would be
+returned by `equalTo`, `isTrue`, etc.).
+
+```php
+$this->assertThat(
+    './tests/actual.png',
+    $this->isSimilarGD('./tests/expected.png')
+);
+```