add bibtex citation code to the README

[nosmokingsurfer]

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}}