Add API Documentation (PDF) link to README
*A lightweight GUI framework suitable for embedded displays*
- [Website (www.impulseadventure.com)](https://www.impulseadventure.com/elec/guislice-gui.html)
- [Documentation wiki (github)](https://github.com/ImpulseAdventure/GUIslice/wiki)
+- [GUIslice API documentation (PDF)](https://github.com/ImpulseAdventure/GUIslice/raw/master/docs/GUIslice_ref.pdf)
- [Release notes](https://github.com/ImpulseAdventure/GUIslice/releases)
## Features ##
