Add docs on customizing string hashing

[caspark]

Closes #89.

It's kinda awkward because I'd like to show how to use FixedState as hasher, but it's not a dependency of ours so the doc sample won't compile (and adding it as a dev dep just for this seems like overkill).

Still, it's better than before, I'd say.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.10%. Comparing base (e92b6b6) to head (8eb940e).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
+ Coverage   60.60%   61.10%   +0.50%     
==========================================
  Files          11       12       +1     
  Lines         500      509       +9     
==========================================
+ Hits          303      311       +8     
- Misses        197      198       +1     

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[Robbepop]

@caspark Can you please fix the doc links as marked by CI?

[caspark]

Should be fixed now.

[Robbepop]

@caspark Thank you for the docs improvements!