Document MetaDiGraph

[caseykneale]

Hey graphs crew. So I am mucking around in your package, and found, by accident you all have implemented MetaDiGraphs! It's not documented. So I am making an issue to remind you all to document it :)

[sbromberger]

That's so weird. Thanks for the heads up.

[caseykneale]

I could contribute some doc strings and things if you want :)

[CameronBieganek]

I also just noticed that there is an undocumented MetaDiGraphs. :)

[sbromberger]

The package needs a bit of love. I've been heads down getting LG 2.0 ready so haven't had much time for the other graph types. Sorry. If someone wants to PR some docs, I'd be happy to review and merge them.