Better document dependencies

[mlabbe]

Currently, building for Linux requires you link libraries. These are undocumented which means you'll just have to compile, receive errors, install libraries and then try again.

[Alloyed]

afaict (and this comes from my luarocks package, which you can use as an example if you want) that just means having gtk 3 installed.
On ubuntu the package you'd want is libgtk-3-dev.
The cflags/ldflags can be gotten from pkg-config, like so:

pkg-config --cflags gtk+-3.0
pkg-config --libs gtk+-3.0

[mlabbe]

Thanks. This is documented in devel.