add "linux" as built platform

README.md

 firmware-freifunk-dresden
 =========================
 
-Original Source / Built tree for <a href="www.freifunk-dresden.de" >Freifunk Dresden</a> Firmware
+Original Source / Built tree for <a href="www.freifunk-dresden.de" >Freifunk Dresden</a> Firmware<br>
+
+Built-Betriebsystem: Linux (e.g.Kubuntu)
 
 Ziel:
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