Update app README template

[mal]

Was updating the readme in my new hubot and realised just how much of it was either out of date or pointing users in the wrong direction.

• Mention ENV vars might need setting for local testing with plugins
• Update shell adapter output
• Clean up some inline linking
• Highlight the shift toward hubot-* plugins (over github/hubot-scripts) and how to find them
  • Purposely haven't mentioned hubot-scripts org after reading hubot-scripts/packages#60
• Consistent text wrapping

[technicalpickles]

Released in v0.2.5, thanks!