docs: Document authentication backend usage

[cdriehuys]

Fixes #43

[codecov]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #47   +/-   ##
=======================================
  Coverage   99.64%   99.64%           
=======================================
  Files          10       10           
  Lines         279      279           
  Branches        8        8           
=======================================
  Hits          278      278           
  Misses          1        1

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