Fix headless Qt license reference in README

Author: johnthagen

README.md

@@ -128,9 +128,9 @@ Third party package licenses are at [LICENSE-3RD-PARTY.txt](https://github.com/s
 
 All wheels ship with [FFmpeg](http://ffmpeg.org) licensed under the [LGPLv2.1](http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html).
 
-Linux wheels ship with [Qt 4.8.7](http://doc.qt.io/qt-4.8/lgpl.html) licensed under the [LGPLv2.1](http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html).
+Non-headless Linux wheels ship with [Qt 4.8.7](http://doc.qt.io/qt-4.8/lgpl.html) licensed under the [LGPLv2.1](http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html).
 
-MacOS wheels ship with [Qt 5](http://doc.qt.io/qt-5/lgpl.html) licensed under the [LGPLv3](http://www.gnu.org/licenses/lgpl-3.0.html).
+Non-headless MacOS wheels ship with [Qt 5](http://doc.qt.io/qt-5/lgpl.html) licensed under the [LGPLv3](http://www.gnu.org/licenses/lgpl-3.0.html).
 
 ### Versioning