Add notes about graphviz and dev.sh in hacking.rst
Add notes about graphviz and dev.sh in hacking.rst

There is a convenient `dev.sh` script in the project root.
This change adds a note about it in the `hacking.rst` file.

It also adds a note about the `graphviz` dependency for building documentation,
and instructions on how to build it.
tych0/qtile,ramnes/qtile,tych0/qtile,qtile/qtile,qtile/qtile,ramnes/qtile