Fix Go dependency install instructions in README

Prior to Go 1.17 the "go get" command was overloaded to install library dependencies and applications. As of Go 1.17 installing applications with it results in a warning, and as of Go 1.18 it will no longer be used to install applications.

This patch updates the readme to use "go install" instead.

TL;DR — "go get" updates module dependencies, "go install" builds binaries and sticks them in $GOBIN.

Signed-off-by: Sam Whited sam@samwhited.com