Document use of go install in readme over go get

[pfiaux]

go install (https://go.dev/ref/mod#go-install) is recommended as the way to install binaries rather than go get.

If I run go get github.com/reugn/wifiqr from the README.md I see the following output:

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'.

What do you think about updating the readme to go install github.com/reugn/wifiqr/cmd/wifiqr@latest, it will install wifiqr to the go bin folder.

Note: I tried go install github.com/reugn/wifiqr@latest but get the following output:

package github.com/reugn/wifiqr is not a main package

Looking at other projects (e.g. https://github.com/knipferrc/fm) a simple main.go at the root should do the trick to make it work.