add info which branch gluon was used

README

@@ -2,4 +2,5 @@ Doku: https://gluon.readthedocs.org/en/v2014.4/user/site.html
 
 Gluon versions used for specific Freifunk Nord Firmware builds:
 
+* 0.7.2: v2015.1.2
 * 0.6: v2014.4