Improved documentation

[orangejulius]

Some of the WOF documentation had grown a little stale.

This PR makes the following changes:

• move configuration options from a table with poor default formatting to individual sections, allowing for much better descriptions
• Correctly describe the importPlace flag's behavior. It has supported multiple values since https://github.com/pelias/whosonfirst/pull/324 but the documentation was not updated
• Reorder the readme to include a Quickstart section near the top, with less frequently needed sections (such as the full list of placetypes) moved lower
• Improve the documentation of the behavior of the downloader when WOF data has not been updated