more detailed README, including div use #14

[jenweber]

Partially addresses #14 and #2

I've made this quite a bit more wordy in the interest of being friendly to new devs. Feel free to request changes or reject certain documentation choices.

The most important are:

• don't say "automatically" because to me that implies I can just install the addon and go about my way, whereas you do need to add the blocks
• explain the div usage
• explain landmarks

[jenweber]

cc @MelSumner