Add the paper to the readme

[Marcin Miłkowski]

I guess academic users might want to cite our work on this?

Miłkowski, M., & Lipski, J. (2011). Using SRX Standard for Sentence Segmentation. In Z. Vetulani (Ed.), Human Language Technology. Challenges for Computer Science and Linguistics (pp. 172–182). Berlin, Heidelberg: Springer Berlin Heidelberg. doi: 10.1007/978-3-642-20095-3_16

I'd add this to the readme ;)

[Jarek Lipski]

Done, thanks.