Compilation instructions out of date (go get has been deprecated)

[sritchie73]

After find that there was no pre-built binary for OSX with arm64 architecture (See issue #58) and no other mechanism for installing on a mac (e.g. via homebrew, see issue #59) I tried following the instructions on the README to compile docopts from source, but recieved the following error message when running make all:

go get github.com/docopt/docopts
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'.
make: *** [install_builddep] Error 1

It appears that go get has been deprecated.

I was eventually able to figure out how to get docopts installed via go install:

go install github.com/docopt/docopts@latest

Which on my machine installed the docopts binary to $HOME/go/bin/