README.md:
* fix GoDoc badge to use pkg.go.dev
* fix "go get" instruction to use module style
* fix build instruction to patch godocdown output to handle how section titles
  are formatted by gofmt in Go 1.18+
* rebuild README.md

Build instructions:

   go install github.com/robertkrimen/godocdown/godocdown@latest
   godocdown -template=.godocdown.template -o README.md
   sed -i= 's/^# /### /' README.md