Adding README file

Author: ramsey

README.md

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+# array_column() for PHP
+
+[![Build Status](https://travis-ci.org/ramsey/array_column.png)](https://travis-ci.org/ramsey/array_column)
+
+This simple library provides functionality for [`array_column()`](https://wiki.php.net/rfc/array_column)
+to versions of PHP earlier than 5.5. It mimics the functionality of the built-in
+function in every way.
+
+
+## Usage
+
+```
+array array_column(array $input, mixed $columnKey[, mixed $indexKey])
+```
+
+Given a multi-dimensional array of data, `array_column()` returns the values
+from a single column of the input array, identified by the `$columnKey`.
+Optionally, you may provide an `$indexKey` to index the values in the returned
+array by the values from the `$indexKey` column in the input array.
+
+For example, using the following array of data, we tell `array_column()` to
+return an array of just the last names, indexed by their record IDs.
+
+``` php
+<?php
+$records = array(
+    array(
+        'id' => 2135,
+        'first_name' => 'John',
+        'last_name' => 'Doe'
+    ),
+    array(
+        'id' => 3245,
+        'first_name' => 'Sally',
+        'last_name' => 'Smith'
+    ),
+    array(
+        'id' => 5342,
+        'first_name' => 'Jane',
+        'last_name' => 'Jones'
+    ),
+    array(
+        'id' => 5623,
+        'first_name' => 'Peter',
+        'last_name' => 'Doe'
+    )
+);
+
+$lastNames = array_column($records, 'last_name', 'id');
+```
+
+If we call `print_r()` on `$lastNames`, you'll see a resulting array that looks
+a bit like this:
+
+``` text
+Array
+(
+    [2135] => Doe
+    [3245] => Smith
+    [5342] => Jones
+    [5623] => Doe
+)
+```
+
+
+## Installation
+
+The easiest way to install this library is to use Composer and add the following
+to your project's `composer.json` file:
+
+``` javascript
+{
+    "require": {
+        "rhumsaa/array_column": "~1.0.0"
+    }
+}
+```
+
+Then, when you run `composer install`, everything will fall magically into place,
+and the `array_column()` function will be available to your project, as long as
+you are including Composer's autoloader.
+
+_However, you do not need Composer to use this library._
+
+This library has no dependencies and should work on older versions of PHP.
+Download the code and include `src/array_column.php` in your project, and all
+should work fine.
+
+When you are ready to run your project on PHP 5.5, everything should
+continue to run well without conflicts, even if this library remains included
+in your project.