Mender just published a Quickstart Guide to the Yocto Project SDK, which can be found under our resources page (click here for the web version). Thank you Scott Rifenbark, who is responsible for much of the Yocto Project's documentation.
While Mender will initially focus on supporting the Yocto Project due to the overwhelming demand from the embedded Linux community, we definitely plan on supporting additional platforms based on community demand. One of our core values is to listen to the needs of the embedded Linux community, so if you want your voice heard we want to hear from you! Please email us at contact@mender.io.