Adding Yunohost Integration level

cf : https://github.com/YunoHost-Apps/zabbix_ynh/pull/3
Thanks @yalh76

README.md

@@ -1,3 +1,5 @@
+[![Integration level](https://dash.yunohost.org/integration/zabbix.svg)](https://ci-apps.yunohost.org/jenkins/job/zabbix%20%28Community%29/lastBuild/consoleFull)  
+[![Install zabbix with YunoHost](https://install-app.yunohost.org/install-with-yunohost.png)](https://install-app.yunohost.org/?app=zabbix)
 > *This package allow you to install zabbix quickly and simply on a YunoHost server.  
 If you don't have YunoHost, please see [here](https://yunohost.org/#/install) to know how to install and enjoy it.*