Add some developer docs

[asinghvi17]

@mkborregaard generated an awesome graph of method calls for histogram here, and it would be nice if we could include that in the docs. We could also look into using D3Trees.jl and building interactive tree visualizations for some plot calls; the code used to generate this PDF is at https://gist.github.com/mkborregaard/81825c3d370bb4d8dbfe59c3b2ae4b33.