Add SoftwareX paper to citation guidelines

Created by: mattpitkin

The GwPy SoftwareX paper should probably be added to the citation guide page. Maybe also include the BibTeX for the paper:

@article{gwpy,
    title = "{GWpy: A Python package for gravitational-wave astrophysics}",
   author = {{Macleod}, D.~M. and {Areeda}, J.~S. and {Coughlin}, S.~B. and {Massinger}, T.~J. and {Urban}, A.~L.},
  journal = {SoftwareX},
   volume = {13},
    pages = {100657},
     year = {2021},
     issn = {2352-7110},
      doi = {10.1016/j.softx.2021.100657},
      url = {https://www.sciencedirect.com/science/article/pii/S2352711021000029},
}