Support for installing on OpenBSD

[leonstafford]

re #141

• builds binaries for OpenBSD during release
• adds notes on installation in README

[codecov]

Codecov Report

Merging #142 into master will increase coverage by 0.43%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
+ Coverage   82.74%   83.17%   +0.43%     
==========================================
  Files          20       20              
  Lines        1153     1153              
==========================================
+ Hits          954      959       +5     
+ Misses        179      174       -5     
  Partials       20       20

Impacted Files	Coverage Δ
htmltest/util.go	55.55% <0%> (+13.88%)	⬆️

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