add bibtex citation code to the README

  Add citation code to be able to put it into bibliography section in latex. For example, the following code can be used, maybe with some minor adjustments:

```
@misc{ferrer_2018,
title={MROB: Mobile Robotics Lab. A CPP-PY library for 3D state estimation}, 
url={https://github.com/MobileRoboticsSkoltech/mrob}, 
journal={GitHub}, 
author={Ferrer, Gonzalo}, 
year={2018}, 
month={Feb}}