Link to v3 version in readme

[janisz]

Looks like our godoc link is obolete and links to 1.2.1 not 3.0.2

[codecov-commenter]

Codecov Report

Merging #313 (17c61ab) into master (77bbae4) will decrease coverage by 0.52%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
- Coverage   89.30%   88.78%   -0.53%     
==========================================
  Files          15       15              
  Lines         767      767              
==========================================
- Hits          685      681       -4     
- Misses         69       72       +3     
- Partials       13       14       +1     

Impacted Files	Coverage Δ
shard.go	86.90% <0.00%> (-1.59%)	⬇️

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