all: remove concept of read-write key

We don't need a read-write key: we can just SSH directly to
jim-backups@backup.jim.sh instead and run commands thay way.
Remove read-write key and document it in the README.

Also add some tools to update the README variables on updates.

Makefile

@@ -38,7 +38,7 @@ rebase:
 	git pull
 	git checkout -
 	git rebase master
-	./initial-setup.sh --update-vars
+	./initial-setup.sh --update-paths
 	systemctl daemon-reload
 	git status