Document how to use the symbol importer

Summary: Some day we might have a fancy daemon for this, but for now at least
provide some instructions on using the existing importers, etc., to index
project symbols.

Test Plan:

• Generated documentation, read over the result.
• Ran the example code.

Reviewers: btrahan, jungejason, davidreuss

Reviewed By: jungejason

CC: aran, jungejason

Maniphest Tasks: T315

Differential Revision: https://secure.phabricator.com/D1262