Add documentation

Author: rtucek

README.md

@@ -10,6 +10,9 @@ Nginx and PHP are compiled in a specific version. Consult the [CHANGELOG.md](./C
 for further version details. [PHP Composer](https://getcomposer.org/) is already installed and ready
 for use.
 
+Xdebug is installed as shared extension, but not activated. Start the container with env
+ENABLE_XDEBUG=1 to enable the extension.
+
 The Nginx and PHP-FPM master processes are controlled by [Honcho](https://github.com/nickstenning/honcho).
 Honcho was chosen over Supervisor, because honcho will exit and kill the container if either Nginx
 or PHP's master process dye for any reason. If you want have to have the container restarted
@@ -42,6 +45,12 @@ Mount your custom `nginx.conf` file at `/usr/local/nginx/conf/nginx.conf`.
 * Mount your custom `www.conf` file at `/usr/local/etc/www.conf`.
 * Mount your custom `php.ini` file at `/usr/local/php/php.ini`.
 
+### Xdebug
+
+Xdebug was installed mainly with the idea of providing code coverage for PHPUnit. Thus it's not
+configured for any specific use case. You can [configure Xdebug](https://xdebug.org/docs/all) via
+`php.ini` if desired.
+
 ### Honcho
 
 If you want to overwrite the default Honcho configuration - mount your custom `Procfile` file at `/`.