Deprecated installation instructions

[achille-roussel]

The installation instructions use go get, which is a deprecated model for install Go programs:

go get: installing executables with 'go get' in module mode is deprecated.
	Use 'go install pkg@version' instead.
	For more information, see https://golang.org/doc/go-get-install-deprecation
	or run 'go help get' or 'go help install'.

However, using go install does not seem to work either due to the "replace" directive in go.mod:

$ go install github.com/segmentio/kubeapply/cmd/kubeapply@latest
go install: github.com/segmentio/kubeapply/cmd/kubeapply@latest (in github.com/segmentio/[email protected]):
	The go.mod file for the module providing named packages contains one or
	more replace directives. It must not contain directives that would cause
	it to be interpreted differently than if it were the main module.