Add pkg.go.dev badge to README

[nyiyui]

A pkg.go.dev badge/hyperlink would be useful when trying to access the documentation on pkg.go.dev.

[codecov-io]

Codecov Report

Merging #76 (132e596) into master (2abe0ee) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff             @@
##            master       #76    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           15        15            
  Lines         1712      1389   -323     
==========================================
- Hits          1712      1389   -323     

Impacted Files	Coverage Δ
jen/do.go	100.00% <0.00%> (ø)
jen/add.go	100.00% <0.00%> (ø)
jen/jen.go	100.00% <0.00%> (ø)
jen/lit.go	100.00% <0.00%> (ø)
jen/tag.go	100.00% <0.00%> (ø)
jen/dict.go	100.00% <0.00%> (ø)
jen/file.go	100.00% <0.00%> (ø)
jen/group.go	100.00% <0.00%> (ø)
jen/custom.go	100.00% <0.00%> (ø)
jen/tokens.go	100.00% <0.00%> (ø)
... and 5 more

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