Update README.md

Add instructions for installing additional plugins

README.md

@@ -36,6 +36,52 @@ In addition to Roundcube core features, the following are made available with th
 
 ![Screenshot of Roundcube](./doc/screenshots/screenshot.png)
 
+## Configuration
+
+You can extend - or even override - the Roundcube configuration which is coming with this package in the file `config/<ROUNDCUBE_DOMAIN>.inc.php`. Do not edit the file `config/config.inc.php` as future upgrades will overwrite it.
+
+#### Multi-users support
+
+* Integrate with YunoHost users and SSO - i.e logout button, YunoHost users search
+
+#### Plugins
+
+You can also install other plugins - which will not be removed with upgrades. To do so, you can use the official [Plugin Repository](https://plugins.roundcube.net/).
+
+##### From the Plugin Repository
+
+Let's say for example that we want to install the [html5_notifier](https://packagist.org/packages/kitist/html5_notifier) plugin.
+
+1. Connect to your server as root using SSH:
+   ```
+   $ ssh admin@1.2.3.4
+   $ sudo -i
+   ```
+
+2. Log in as the `roundcube` user - which owns the roundcube directory - and navigate in it:
+   ```
+   # su -s /bin/bash - roundcube
+   $ cd /var/www/roundcube
+   ```
+
+3. Install the plugin you want using composer - note that you have to specify *kitist/html5_notifier* and not only *html5_notifier*:
+   ```
+   $ COMPOSER_HOME=./.composer php composer.phar require "kitist/html5_notifier"
+   ```
+
+4. Enable it in the local configuration file `config/<DOMAIN>.inc.php` using your favorite text editor by adding:
+   ```
+   <?php
+   array_push($this->prop['plugins'], 'html5_notifier');
+   ```
+   See https://github.com/roundcube/roundcubemail/issues/9458.
+
+Note that you should also check the plugin homepage for additional installation steps as needed.
+
+##### Manual installation
+
+You can also download the plugin and put it under the `plugins/` directory. In this case, do not forget to change ownerships of this folder to `roundcube`.
+
 ## Documentation and resources
 
 - Official app website: <https://roundcube.net/>