README.md: don't recommend 'go get'

With the latest version of go, the recommended 'go get' commands fail:

    go: go.mod file not found in current directory or any parent directory.
            'go get' is no longer supported outside a module.
            To build and install a command, use 'go install' with a version,
            like 'go install example.com/cmd@latest'
            For more information, see https://golang.org/doc/go-get-install-deprecation
            or run 'go help get' or 'go help install'.

Replace them with commands that work.

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