Update Readme

[JakeWharton]

• New GIF
• Update mention of MockGalleryService
• Ensure library list is up-to-date
• Describe flavors

[JakeWharton]

Re-frame the summary. This is no longer really a Dagger sample. But "how" I/we (mostly) build apps and a complex integration sample for open source libraries which are mine, Square's on which I work, or ones I/we like.