Add BibTeX entry Henlein, Kurz 2017

Dear Sir or Madam,

Thank you for the interesting catalogue of hard MILP instances. I contributed

c38db0fc

to the 2017 set of instances. In the meantime, the paper which contains the background of the importance of these problems was published in arXiv. Hence I kindly ask you to update this contribution by adding the following BibTex reference:

@article{heinlein2017new, title={A new upper bound for subspace codes}, author={Heinlein, Daniel and Kurz, Sascha}, journal={arXiv preprint arXiv:1703.08712}, year={2017} }

Thank you in advance.

Yours faithfully Daniel Heinlein