Add standalone Makefile for the examples

We want to allow people to build the examples easily, without
necessarily requiring them to build the whole of GTK+.

https://bugzilla.gnome.org/show_bug.cgi?id=741041
docs: Add a README to the examples directory

https://bugzilla.gnome.org/show_bug.cgi?id=741041
docs: Mention the standalone Makefile for examples

And point to the README file.

https://bugzilla.gnome.org/show_bug.cgi?id=741041