DOC: add citation file for GitHub support.
DOC: add citation file for GitHub support.

[SKIP CI]

numpy/numpy,pdebuyl/numpy,jakirkham/numpy,mhvk/numpy,pdebuyl/numpy,jakirkham/numpy,charris/numpy,endolith/numpy,mattip/numpy,mattip/numpy,jakirkham/numpy,mhvk/numpy,simongibbons/numpy,anntzer/numpy,pdebuyl/numpy,mhvk/numpy,rgommers/numpy,rgommers/numpy,mhvk/numpy,mhvk/numpy,endolith/numpy,simongibbons/numpy,seberg/numpy,rgommers/numpy,mattip/numpy,endolith/numpy,charris/numpy,seberg/numpy,numpy/numpy,numpy/numpy,numpy/numpy,endolith/numpy,simongibbons/numpy,seberg/numpy,jakirkham/numpy,mattip/numpy,simongibbons/numpy,charris/numpy,anntzer/numpy,seberg/numpy,simongibbons/numpy,charris/numpy,jakirkham/numpy,pdebuyl/numpy,rgommers/numpy,anntzer/numpy,anntzer/numpy