Add a command to update the Erlang.mk guide online

"make up" will be how it's done from now on, rather than the
more manual method I was using until now.

I am fully aware that this command fetches the erlang.mk repo
twice. But it works, and honestly fetching the repo isn't too
expensive or long so that will do.

Makefile

@@ -61,3 +61,10 @@ clean:
 
 docs:
 	$(MAKE) -f core/core.mk -f core/docs.mk -f plugins/asciidoc.mk asciidoc
+
+up:
+	git clone git@github.com:ninenines/erlang.mk.git gh-pages
+	cd gh-pages && git checkout gh-pages
+	cd gh-pages && make
+	cd gh-pages && git push origin gh-pages
+	rm -rf gh-pages