UPGRADING: Add release notes for 1.9.1

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>

UPGRADING

 Upgrading
 =========
 
+From 1.9.0 to 1.9.1
+-------------------
+
+1. Merge "web/lib/config.inc.php.proto" with "web/lib/config.inc.php".
+
+2. Install translations by running `make install` in "po/".
+
 From 1.8.2 to 1.9.0
 -------------------