Add instructions how to add a new model

[schillic]

No description provided.

[mforets]

What did we want to do here, more exactly? Some general instructions on adding a model to ReachabilityBenchmarks?

[schillic]

I think just explain how to construct a model (both continuous and hybrid). Maybe with the option to define a matrix manually and with loading a .mat file.
(We could just copy an existing model and add some comments.)

[mforets]

Shall we add this as a section in the Reachability documentation? I think that we can split between linear / nonlinear and continuous / hybrid.

[schillic]

Yes, I think we can do that and then link to that part of the manual.