README update needed -- Installation generates an error since go get is deprecated outside of modules.

[neoh4x0r]

The command

$ go get github.com/ericchiang/pup

Has been deprecated and will generate the folllowing error:

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'.

For newer versions of go the following command should be used instead:

$ go install github.com/ericchiang/pup@latest

Another related issue was opended (about updating README.md) #145