Create a Makefile and add usage information in README.md

I forgot to change the MODDIR variable before I made the first commit which is why there is a third. I was using a sandboxed `.znc/modules` directory to test the Makefile. As a result of this there is also a minor typo in the `README.md` but I don't feel that is worth a fourth commit. I will make sure that gets corrected in a future commit though.
 
In the near future I would like to improve this Makefile to allow users to clean (remove) modules without removing __all__ of the `.so` files in the `~/.znc/modules` directory.