Update README

README.md

@@ -73,7 +73,9 @@ If you wish to work on Packer itself, you'll first need [Go](http://golang.org)
 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).
 
-Additionally, to build Packer, you will need [Mercurial](http://mercurial.selenic.com/) installed.
+For some additional dependencies, Go needs [Mercurial](http://mercurial.selenic.com/)
+to be installed. Packer itself doesn't require this but a dependency of a
+dependency does.
 
 Next, clone this repository then just type `make`. In a few moments,
 you'll have a working `packer` executable: