use https repo url to clone for dev setup instructions

the git clone line in the dev setup instructions does not work as is, unless the user has write access

README.md

@@ -269,7 +269,7 @@ export PATH=$GOPATH/bin:$PATH
 
 mkdir -p $GOPATH/src/github.com/dotcloud
 cd $GOPATH/src/github.com/dotcloud
-git clone git@github.com:dotcloud/docker.git
+git clone https://github.com/dotcloud/docker.git
 cd docker
 
 go get -v github.com/dotcloud/docker/...