Copter: update release notes for AC3.1.5-rc2

ArduCopter/ReleaseNotes.txt

 ArduCopter Release Notes:
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+ArduCopter 3.1.5-rc2 20-May-2014
+Changes from 3.1.5-rc1
+1) Bug Fix to broken loiter (pixhawk only)
+2) Workaround to read from FRAM in 128byte chunks to resolve a few users boot issues (Pixhawk only)
+------------------------------------------------------------------
 ArduCopter 3.1.5-rc1 14-May-2014
 Changes from 3.1.4
 1) Bug Fix to ignore roll and pitch inputs to loiter controller when in radio failsafe