Update installation instructions for latest Go version

[justincampbell]

go install is now required when not running from within a module, and the subcommand path is required as the root is not a main package.

[hmarr]

That line actually refers to how to install this module as a library rather than the CLI tool, but I agree it's probably a bit confusing to have them next to each other.

Can I propose a slightly bigger change instead?

• Have two <h2> level headings — "CLI tool" and "Go library"
• Under each of those, have <h3> headings for "Installation" and "Usage"
• Under the "Installation" heading for the CLI section, show both the homebrew install instructions and "from source" install instructions using go install
• Move the go get command to the "Go library" section

WDYT?

[justincampbell]

That sounds good! Feel free to take merge or close this and do whichever you wish. I only submitted the change because the other instructions didn't work when I tried to install it for CLI usage. Thanks!