Updated the README.
Updated the README.

The README now contains a mention of the used license as well as a simple
example of how to use the Gem.

Signed-off-by: Yorick Peterse <82349cb6397bb932b4bf3561b4ea2fad50571f50@gmail.com>

YorickPeterse/ruby-gtk3,YorickPeterse/ruby-gtk3