Add section describing how app works to index route

[markmhendrickson]

This entails adding an informational section to the index route for unauthenticated visits that explains how the app works.

Alternatively, this section could be added to its own route placed within an onboarding flow initiated from the index route.

Possible information to include:

• We don't request or gain full access to storage, just single folders
• We don't store users' source data on our servers except for light amounts of meta data