Added reference to PostgreSQL store implementation

[vgarvardt]

I implementad PostgreSQL token and clients store with support for different golang/pg drivers for one of my projects. https://github.com/vgarvardt/go-oauth2-pg

[codecov-io]

Codecov Report

Merging #95 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #95   +/-   ##
=======================================
  Coverage   52.81%   52.81%           
=======================================
  Files          11       11           
  Lines         888      888           
=======================================
  Hits          469      469           
  Misses        313      313           
  Partials      106      106

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