Expand README to include build and running example instructions

[dov]

Currently the README is very short.

Currently building is trivial through meson, but adding a few sentences about it wouldn't hurt.

Running the examples doesn't work out of the box. E.g. trying to run ./build/examples/cubes-gtk3 from the main directory fails with the error:

** (cubes-gtk3:36003): ERROR **: 20:18:53.885: could not load crate.gif
[1]    36003 trace trap (core dumped)  ./build/examples/cubes-gtk3

[dov]

To answer my own question and after delving into the meson.bulid file. To run the examples do the following:

meson --reconfigure --prefix=/tmp/local build
ninja -C build install

[giuliohome]

Thank you @dov , that is helpful indeed!

BTW I’ve also changed meson default option from gtk3 to gtk4