README.asciidoc: add link to 'u8glib'
README.asciidoc: add link to 'u8glib'

u8glib is a "Universal Graphics Library for 8 Bit Embedded Systems". It
looks very similar to what I want fgui to be. There is also a GUI (named
M2tklib) built on top of the graphics primitives that u8glib provides.

Must try!

bjornfor/fgui