Add some design goals in the README.
Add some design goals in the README.

peterhil/impressionist,bmcmurray/hekyll,flibbertigibbet/hekyll,peterhil/impressionist,bmcmurray/hekyll,bmcmurray/hekyll,peterhil/impressionist,flibbertigibbet/hekyll,flibbertigibbet/hekyll