Add citation for HomotopyContinuation

This one is kinda niche since most users probably won't use this functionality. However, we should have it somewhere for people to get if they do use it.

Author: TorkelE

CITATION.bib

@@ -11,3 +11,12 @@ @article{DifferentialEquations.jl-2017
  volume = {5},
  year = {2017}
 }
+
+@inproceedings{HomotopyContinuation.jl,
+  title={HomotopyContinuation.jl: A Package for Homotopy Continuation in Julia},
+  author={Breiding, Paul and Timme, Sascha},
+  booktitle={International Congress on Mathematical Software},
+  pages={458--465},
+  year={2018},
+  organization={Springer}
+}