Start Entity Relationship diagram for Microsoft Academic API

Source:
https://datamarket.azure.com/dataset/mrc/microsoftacademic#schema

The description of the Entity Relationship diagram uses a text
format, then converted to an image with the tool 'erd'.

Project home page:
https://github.com/BurntSushi/erd

On Mac OS X, with XCode already installed, I had to install
two dependencies first:  Haskell and GraphViz.

I then installed erd from the command line using cabal,
part of Haskell:

    cabal install erd

And I added erd to the path in the shell by adding the following
line to my .bash_profile:

    export PATH="$HOME/Library/Haskell/bin:$PATH"

I then cloned the erd repository and tested my setup by converting
one of the examples to a PNG:

    erd -i erd/examples/nfldb.er -o nfldb.png

Which failed at first, because I had not installed GraphViz at
this point. I then installed GraphViz and the transformation
worked with no need to run the setup again.