Add notes for preprints in docs

[sampan501]

Reference issue

Type of change

What does this implement/fix?

Additional information

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[codecov]

Codecov Report

Base: 96.25% // Head: 96.25% // No change to project coverage 👍

Coverage data is based on head (995cfe3) compared to base (8bf37e3).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #338   +/-   ##
=======================================
  Coverage   96.25%   96.25%           
=======================================
  Files          47       47           
  Lines        2296     2296           
=======================================
  Hits         2210     2210           
  Misses         86       86           

Impacted Files	Coverage Δ
hyppo/conditional/FCIT.py	97.36% <ø> (ø)
hyppo/d_variate/dhsic.py	100.00% <ø> (ø)
hyppo/independence/friedman_rafsky.py	100.00% <ø> (ø)
hyppo/independence/kmerf.py	100.00% <ø> (ø)
hyppo/independence/max_margin.py	100.00% <ø> (ø)
hyppo/kgof/fssd.py	97.38% <ø> (ø)
hyppo/ksample/ksamp.py	92.68% <ø> (ø)
hyppo/time_series/dcorrx.py	100.00% <ø> (ø)
hyppo/time_series/mgcx.py	100.00% <ø> (ø)
hyppo/conditional/kci.py	97.72% <100.00%> (ø)

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