Add some basic documentation for Legalpad

Summary: Ref T3116. Explain a couple of core use cases and contextualize the app a bit.

Test Plan: Read application help screen and user guide.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D9777