Add documentation for custom formatters

[chreke]

Great work on this library, it's really cool!

I was looking for a way to add custom formatters and found the custom_formatters argument. I found it very useful, so I thought I'd add it to the documentation.

I'm not sure if the usage example I gave is the best (are you supposed to provide both validate and unmarshal in this case?) so I'd be happy to get suggestions on how to improve it.

I didn't find any contribution guidelines so I hope I'm doing this right!

Best regards,
@chreke

[codecov]

Codecov Report

Merging #228 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #228   +/-   ##
=======================================
  Coverage   96.79%   96.79%           
=======================================
  Files         123      123           
  Lines        2403     2403           
=======================================
  Hits         2326     2326           
  Misses         77       77           

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[p1c2u]

Hi @chreke

thank you for the contribution. LGTM 👍