Installing addons with no distribution

When I'm developing a new add-on or adopting one that's not in (like, for example, collective.saml2), how do I add those to my buildout? Just adding the egg to the [buildout] section results in the error Couldn't find a distribution for collective.saml2 (or whatever the name of the addon is).

Use the mr.developer buildout extension and add them as source (when used for production you want to pin them to specific tags/branches/commits).