Install notes out of date

[digininja]

The install notes here:

https://revel.github.io/tutorial/gettingstarted.html

Are out of date. When you try to use the go get command to install revel it fails due to changes in the way modules are installed.

$ go get github.com/revel/revel
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'.