Updated the README.md file.

Author: BluePsyduck

README.md

@@ -1,5 +1,66 @@
 # Symfony Process Manager
 
-[![Latest Stable Version](https://poser.pugx.org/bluepsyduck/symfony-process-manager/v/stable)](https://packagist.org/packages/bluepsyduck/symfony-process-manager) [![License](https://poser.pugx.org/bluepsyduck/symfony-process-manager/license)](https://packagist.org/packages/bluepsyduck/symfony-process-manager) [![Build Status](https://travis-ci.com/BluePsyduck/symfony-process-manager.svg?branch=master)](https://travis-ci.com/BluePsyduck/symfony-process-manager) [![codecov](https://codecov.io/gh/BluePsyduck/symfony-process-manager/branch/master/graph/badge.svg)](https://codecov.io/gh/BluePsyduck/symfony-process-manager)
+[![Latest Stable Version](https://poser.pugx.org/bluepsyduck/symfony-process-manager/v/stable)](https://packagist.org/packages/bluepsyduck/symfony-process-manager)
+[![License](https://poser.pugx.org/bluepsyduck/symfony-process-manager/license)](https://packagist.org/packages/bluepsyduck/symfony-process-manager)
+[![Build Status](https://travis-ci.com/BluePsyduck/symfony-process-manager.svg?branch=master)](https://travis-ci.com/BluePsyduck/symfony-process-manager)
+[![codecov](https://codecov.io/gh/BluePsyduck/symfony-process-manager/branch/master/graph/badge.svg)](https://codecov.io/gh/BluePsyduck/symfony-process-manager)
 
-TBD
+This package provides a simple process manager class to be able to execute multiple processes with a specified limit
+of parallel processes. The class expects the processes to use the [Symfony Process](https://github.com/symfony/process) 
+component.
+
+## Usage
+
+The usage of the process manager is straight forward and best explained with an example.
+
+```php
+<?php
+use BluePsyduck\SymfonyProcessManager\ProcessManager;
+use Symfony\Component\Process\Process;
+
+$numberOfParallelProcesses = 4; // The number of processes to execute in parallel.
+$pollInterval = 100; // The interval to use for polling the processes, in milliseconds.
+
+$processManager = new ProcessManager($numberOfParallelProcesses, $pollInterval);
+
+// Add some processes
+// Processes get executed automatically once they are added to the manager. 
+// If the limit of parallel processes is reached, they are placed in a queue and wait for a process to finish.
+$processManager->addProcess(new Process('ls -l'));
+$processManager->addProcess(new Process('ls -l'));
+
+// Wait for all processes to finish
+$processManager->waitForAllProcesses();
+
+
+```
+
+## Callbacks
+
+The process manager allows for some callbacks to be specified, which get called depending on the state of a process.
+
+* **processStartCallback:** Triggered before a process is started.
+* **processFinishCallback:** Triggered when a process has finished.
+* **processSuccessCallback:** Triggered when a processes has finished with an exit code of 0.
+* **processFailCallback:** Triggered when a processes has failed with an exit code other than 0.
+
+_Note:_ Each process will trigger either the `processSuccessCallback` or the `processFailCallback` depending of its exit 
+code. The `processFinishCallback` will always be triggered afterwards, ignoring the exit code.
+
+Each callback gets the process instance which triggered the event passed as only parameter. Here is an example of 
+setting a `processStartCallback`:
+
+```php
+<?php
+use BluePsyduck\SymfonyProcessManager\ProcessManager;
+use Symfony\Component\Process\Process;
+
+$processManager = new ProcessManager();
+
+$processManager->setProcessStartCallback(function (Process $process): void {
+    echo 'Starting process: ' . $process->getCommandLine();
+});
+
+$processManager->addProcess(new Process('ls -l'));
+$processManager->waitForAllProcesses();
+```