Update the README with some absolute paths

The "quick install" line used to suggest the user to `git clone --recursive` into `.emacs.d`. Perhaps is better to make the user clone in `~/.emacs.d` so that they can install Spacemacs whatever their `$(pwd)` is.

I'm committing this since a couple of times I was frustrated that the Spacemacs installation didn't do anything just because I cloned `.emacs.d` somewhere other than `~`.