Add NWB citation to README

[oruebel]

Fix #1322

Checklist

• Have you checked our Contributing document?
• Have you ensured the PR description clearly describes problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using #XXX notation where XXX is the issue number?

[codecov]

Codecov Report

Merging #1323 (6ca11c5) into dev (13b6c4e) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1323      +/-   ##
==========================================
+ Coverage   73.41%   73.49%   +0.08%     
==========================================
  Files          37       37              
  Lines        2328     2328              
  Branches      357      357              
==========================================
+ Hits         1709     1711       +2     
+ Misses        557      556       -1     
+ Partials       62       61       -1     

Impacted Files	Coverage Δ
src/pynwb/icephys.py	90.08% <ø> (ø)
src/pynwb/misc.py	87.31% <ø> (ø)
src/pynwb/file.py	82.75% <100.00%> (+0.62%)	⬆️

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[rly]

Closed in favor of #1362 which cites the most recent NWB preprint