README: Remove deprecated install target and check for go in setup

seems like i removed that target at some point and forgot to update the
readme. As installing go is os specific let's just check for go instead and
let the user handle the installing.

Makefile

@@ -16,6 +16,7 @@ test:
 .PHONY: setup
 setup:
 	git config core.hooksPath etc/githooks
+	command -v go > /dev/null || (echo "go not installed" && false)
 
 .PHONY: preview
 preview: generate

README.org

@@ -27,7 +27,7 @@ Commands:
 ** as a library
 see [[https://github.com/niklasfasching/go-org/blob/master/main.go][main.go]] and hugo [[https://github.com/gohugoio/hugo/blob/master/markup/org/convert.go][org/convert.go]]
 * development
-1. =make setup install=
+1. =make setup=
 2. change things
 3. =make preview= (regenerates fixtures & shows output in a browser)