Adding `go install` to the installation instructions in README.md

[krader1961]

I was modifying the Elvish shell to use glow to dynamically generate a Go source file that contains documentation for the shell's builtin functions and variables. I ultimately decided that the simplest, most future proof, mechanism (given the Elvish is also written in Go and hosted on Github) was to modify its CI workflow with these lines:

- name: Install glow to generate the documentation for the "help" command
   run: go install github.com/charmbracelet/glow@latest

As opposed to downloading a pre-built binary. Something like the above would be a useful addition to the README for installing glow as an alternative, or even replacement for, the git clone ...; go build ... sequence.