Reflex-starterApp: happy needs to be on the $PATH

[pheara]

I tried to just build the ghcjs/reflex starter app (this is the first working build setup I've found so far ^^), and ran into the problem of it requiring happy. A global stack install happy fixed it for me . A comment in #595 of the stack project also mentions stack build <build-tool> in the project folder to be viable (where build-tool = happy).

Dunno if i missed this in the documentation elsewhere, otherwise I could open a pull-request for the starter-app's README.org.