Add instruction on version upgrade
Add instruction on version upgrade

"./gnirehtet rt" does not automatically install the apk on the device if
the package is already present.

As a consequence, it must be explicitly installed on upgrade.

JonnyShuali/gnirehtet,Genymobile/gnirehtet,WPO-Foundation/gnirehtet,JonnyShuali/gnirehtet,Genymobile/gnirehtet,WPO-Foundation/gnirehtet,WPO-Foundation/gnirehtet,Genymobile/gnirehtet,WPO-Foundation/gnirehtet