add travis-ci and appveyor build status

README.md

 # QGroundControl
 ## Open Source Micro Air Vehicle Ground Control Station
 
+[![Travis Build Status](https://travis-ci.org/mavlink/qgroundcontrol.svg?branch=master)](https://travis-ci.org/mavlink/qgroundcontrol)
+[![Appveyor Build Status](https://ci.appveyor.com/api/projects/status/crxcm4qayejuvh6c/branch/master?svg=true)](https://ci.appveyor.com/project/mavlink/qgroundcontrol)
+
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