Document `doc` registration

[phreed]

In the index.md the following lines appear.

After installation, verify installation by executing dot -V, example:

dot - graphviz version 2.38.0 (20140413.2041)

These instructions are fine as far as they go but it may also be necessary to register the layout engines.
This can be done with the following command (immediately after verifying that dot is installed).

dot -V
dot -c