Added a basic makefile with some helpful commands.

[elland]

Not sure how much that makes sense for others. Feel free to reject.

[david-christiansen]

I'm afraid that the Makefile adds more confusion than it removes for new contributors. There's already two separate build techniques in the README, and adding a third seems like a bit much, esp. for new Haskellers who might go for make by default.

[mknorps]

I am afraid that it is hard to do one common script for different users. For example I am using stack, and not cabal commands.

[david-christiansen]

I think I'm going to close this one - I think the potential for confusion is higher than the likelihood of it being helpful for new contributors. But thanks for sharing it!