Update README to note GOPATH [GH-71]

README.md

@@ -70,8 +70,11 @@ http://www.packer.io/docs
 ## Developing Packer
 
 If you wish to work on Packer itself, you'll first need [Go](http://golang.org)
-installed (version 1.1+ is _required_). Next, clone this repository then just type `make`.
-In a few moments, you'll have a working `packer` executable:
+installed (version 1.1+ is _required_). Make sure you have Go properly installed,
+including setting up your [GOPATH](http://golang.org/doc/code.html#GOPATH).
+
+Next, clone this repository then just type `make`. In a few moments,
+you'll have a working `packer` executable:
 
 ```
 $ make