Minor deployment instruction improvements

This PR implements README improvements based on observing @migurski deploy and seeing the pain points. 

Among the improvements:

1. Added `git clone` instruction
2. Moved up 3rd party services and noted costs
3. Provided alternative approach if `rake secret` fails
4. Reworded "Basic auth" to be more plain-spoken (password-protection)
5. Added Twilio config screenshot
6. Clarified what services are optional

Note that this PR does *not* include instructions on configuring a custom survey/locations just yet — this is intended to solely focus on improving the existing README experience.

cc @migurski for review