Add README

Author: timvdalen

README.md

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+Redis counting semaphore [![Latest Stable Version](https://poser.pugx.org/code-orange/redis-counting-semaphore/v/stable)](https://packagist.org/packages/code-orange/redis-counting-semaphore) [![Total Downloads](https://poser.pugx.org/code-orange/redis-counting-semaphore/downloads)](https://packagist.org/packages/code-orange/redis-counting-semaphore) [![License](https://poser.pugx.org/code-orange/redis-counting-semaphore/license)](https://packagist.org/packages/code-orange/redis-counting-semaphore) [![composer.lock](https://poser.pugx.org/code-orange/redis-counting-semaphore/composerlock)](https://packagist.org/packages/code-orange/redis-counting-semaphore)
+========================
+
+`redis-counting-semaphore` is a package with a counting semaphore implementation for PHP.
+It uses redis as a central broker.
+
+# Installation
+
+To install redis-counting-semaphore with composer:
+
+```
+composer require code-orange/redis-counting-semaphore
+```
+
+# Usage
+
+First, make sure you have a [Predis](https://github.com/nrk/predis) connection instance.
+
+You can create and attempt to obtain a semaphore like so:
+ 
+```php
+<?php
+use CodeOrange\RedisCountingSemaphore\Semaphore;
+
+$client = new Predis\Client();
+
+// Create a counting semaphore with a limit of 3
+$sem = new Semaphore($client, 'semaphore-name', 3);
+if ($sem->acquire(0.1, 10)) {
+	// Obtained the semaphore
+	use_limited_resource();
+	$sem->release();
+} else {
+	// We weren't able to get a semaphore, even though we tried 10 times
+	// And slept for 0.1 seconds in between tries
+}
+```
+
+## API
+
+```php
+/**
+ * Semaphore constructor.
+ * @param Client $client Predis client with an open connection
+ * @param string $name Name of the semaphore
+ * @param int $limit The amount of resources this semaphore protects
+ * @param int $timeout Timeout of an acquired semaphore, in seconds
+ */
+public Semaphore(Client $client, $name, $limit = 1, $timeout = 10);
+
+/**
+ * Try to acquire a semaphore
+ *
+ * @param float $sleep Number of seconds to sleep between retries. If null, this function will not retry but return immediately.
+ * @param int $retries Number of times to retry before giving up
+ * @return bool Whether or not the semaphore was acquired correctly
+ */
+public function acquire($sleep = null, $retries = null);
+
+/**
+ * Release this semaphore
+ *
+ * @return void
+ */
+public function release();
+
+/**
+ * Refresh the semaphore
+ *
+ * @return bool Whether or not we still have the semaphore
+ */
+public function refresh();
+```