thermostat: update INSTALL with some help on translation

soft/thermostat/INSTALL

@@ -5,3 +5,7 @@ cd qtmqtt
 git checkout v$(qmake -v | grep "Qt version" | sed "s/^.*version \([5-9]\.[0-9]\+\.[0-9]\+\) .*$/\1/") || echo "Unable to find your Qt version"
 qmake && make -j$(nproc)
 sudo make install
+
+Update/edit translations:
+lupdate -verbose thermostat.pro
+linguist lang/thermostat_fr.ts