>> * .gitignore: Add `dash'.
> Is this supposed to be done for all external packages? If so, we should add
> that to the readme.
Yes. I recommend you do:
git clone .../elpa.git
cd elpa
make externals
make
and then add that ../elpa/packages directory to package-directory-list.
At some point hopefully, the above would be better integrated with
Emacs's own build procedure so it can be done more automatically, thus
reducing further the difference between packages included in Emacs and
those in GNU ELPA.
Stefan