Add docs section about custom plots

[fbanning]

This docs section is added in response to JuliaDynamics/Agents.jl#551.

@Datseris I would like to add two screenshots to this section. Where would you normally store those? I saw that you've stored the videos for the docs landing page on the JuliaDynamics github space.

[Datseris]

Hi @fbanning cool! Yeap we store "external" images here https://github.com/JuliaDynamics/JuliaDynamics/tree/master/videos/agents in order to minimize download size!

[fbanning]

I don't need to bump the patch number for a docs update, right?

[Datseris]

for the change to be reflected in the stable docs yes. However at the moment master branch is unstable because graph space uses Graphs.jl and OSM uses LightGraphs.jl. So don't bother with bversions!

[fbanning]

at the moment master branch is unstable

You might be thinking about Agents right now. As far as I can tell, InteractiveDynamics does not have anything specifically unstable in its current master branch. If you're OK with my additions in this PR, I would say you could already merge it.

[Datseris]

It is getting really hard to keep track of many large projects.... :D