Add continuous integration build badges

Added badges for build status of Travis CI and
AppVeyor to the front page README, giving a
quick overview.

README.md

@@ -1,6 +1,9 @@
 MilkyTracker - Cross-Platform XM Tracker
 ========================================
 
+[![Travis Build Status](https://travis-ci.org/milkytracker/MilkyTracker.svg?branch=master)](https://travis-ci.org/milkytracker/MilkyTracker)
+[![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/github/milkytracker/MilkyTracker?branch=master&svg=true)](https://ci.appveyor.com/project/Deltafire/milkytracker)
+
 MilkyTracker is an multi-platform music application for creating .MOD
 and .XM module files. It attempts to recreate the module replay and
 user experience of the popular DOS program Fasttracker II, with