Adds the CITATION file for those who would like to cite h5py in their scientific publications

[erikhuck]

First off let me thank the maintainers of this amazing project. It has been so helpful to me personally in my research and I'm certain it helps a lot of other researchers as well, and it deserves proper recognition in any scientific publications that uses it.

This PR is in response to #743 . I think this should provide a straightforward way to cite h5py. If the maintainers of the project prefer a different way of citing h5py, such as citing the package itself, I hope this starts that conversation.

Fixes #743

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.53%. Comparing base (b6035fb) to head (af9aa61).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2393   +/-   ##
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  Coverage   89.53%   89.53%           
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  Files          17       17           
  Lines        2380     2380           
=======================================
  Hits         2131     2131           
  Misses        249      249           

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