Added Upgrade Messages section to the read me Fixes IOS-264
Added Upgrade Messages section to the read me
Fixes IOS-264

apptentive/apptentive-ios,ALHariPrasad/apptentive-ios,apptentive/apptentive-ios,Jawbone/apptentive-ios,Jawbone/apptentive-ios,hibu/apptentive-ios,sahara108/apptentive-ios,sahara108/apptentive-ios,apptentive/apptentive-ios,hibu/apptentive-ios,ALHariPrasad/apptentive-ios,hibu/apptentive-ios