Merge pull request #650 from mitchellh/readme-gox

readme: add a note about the gox dependency for compiling

README.md

@@ -77,6 +77,13 @@ For some additional dependencies, Go needs [Mercurial](http://mercurial.selenic.
 and [Bazaar](http://bazaar.canonical.com/en/) to be installed.
 Packer itself doesn't require these, but a dependency of a dependency does.
 
+You'll also need [`gox`](https://github.com/mitchellh/packer)
+to compile packer. You can install that with:
+
+```
+$ go get -u github.com/mitchellh/gox
+```
+
 Next, clone this repository into `$GOPATH/src/github.com/mitchellh/packer` and
 then just type `make`. In a few moments, you'll have a working `packer` executable: