add a comment linking to xvfb documentation

ci/test_unittests.sh

@@ -22,6 +22,7 @@ cd build-ci/dashcore-$BUILD_TARGET
 
 if [ "$RUN_TESTS" = "true" -a "${DEP_OPTS#*NO_QT=1}" = "$DEP_OPTS" ]; then
   export DISPLAY=:99.0;
+  # Start xvfb if needed, as documented at https://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-xvfb-to-Run-Tests-That-Require-a-GUI
   /sbin/start-stop-daemon --start --pidfile /tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -ac;
 fi