Add AppImageKit to Freezing tools section

[probonopd]

AppImageKit can be used to bundle the Python interpreter, libraries, modules, the payload Python application and any resources such as icons, fonts, graphics, etc., into a single executable file for Linux. The single executable file is loop-mounted when executed using FUSE, and the payload Python interpreter is executed.

http://appimage.org/

https://github.com/probonopd/AppImages/issues/172