docs: Add installation with go install

[mloskot]

I'm a Go newbie who followed the GOPATH installation from within $HOME on Linux machine, and I got this:

$ cd ~ 
$ go get -u -d github.com/magefile/mage
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'

Then, I tried the suggestion:

$ go install github.com/magefile/mage@latest
go: downloading github.com/magefile/mage v1.15.0

which was simplest, clear and worked like a charm:

$ mage --version
Mage Build Tool <not set>
Build Date: <not set>
Commit: <not set>
built with: go1.20.4

so, IMHO, the go install way should be promoted as the recommended installation of mage tool.

[mloskot]

which was simplest, clear and worked like a charm:

Ha! I've just re-read the README.md and this part

A normal go get (without -d) or go install will build the binary correctly, but no version info will be embedded.

and the version is not embedded indeed. How could have I missed that! 🤦‍♂️