Update README.md

Author: Marius Burkard

README.md

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 # ISPConfig - Hosting Control Panel
 
+Nightly (master): [![pipeline status](https://git.ispconfig.org/ispconfig/ispconfig3/badges/master/pipeline.svg)](https://git.ispconfig.org/ispconfig/ispconfig3/commits/master)   
+Stable branch: [![pipeline status](https://git.ispconfig.org/ispconfig/ispconfig3/badges/stable-3.1/pipeline.svg)](https://git.ispconfig.org/ispconfig/ispconfig3/commits/stable-3.1)
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 - Manage multiple servers from one control panel
 - Web server management (Apache2 and nginx)
 - Mail server management (with virtual mail users)