Document how to get cbcflow-data for running the tests

[Simon Funke]

I did not find any explicit instructions on how to get the cbcflow-data for running the tests.

I suggest that you add the cbcflow-data repo as an dependency to cbcflow repo via a git submodule (see http://git-scm.com/docs/git-submodule).

Then downloading the cbcflow-data would become:

git submodule init
git submodule update