Improve welcome onboard message

[barbeau]

Summary:

From user feedback in the beta (#65) - after setting up OBA Alexa with a city and stop, we should give clearer information for how the skill can be invoked in the future, and immediately read off the current arrival info for that stop.

Steps to reproduce:

Set the city to any city (e.g., Tampa), and any stop number (e.g., 6497).

Expected behavior:

Here's feedback from users from the beta (#65):

• After setting a stop number, the skill responds with "Ok, your stop number is 75414 in the Puget Sound region. Great. I am ready to tell you about the next bus." and the stream closes. Ideally, the skill would go right in to telling the next bus information or prompt users to ask for this information while leaving the stream open so the skill would not have to be relaunched.
• Lastly, after I set up, I didn’t know what to do. I can’t remember what the skills said. Something like “now your location is set.” and then that was it. I wanted to hear something like, “Now your location is set. To find out bus schedule information, just say “Alexa, open OneBusAway” or whatever other common terms will work.
• After giving the city and stop number, Alexa said that she was ready to help but then the session ended. I was expecting she would either let me ask “when is the next bus arriving”, or that she would provide some instructions on how to ask for information in the future, like the types of info devs provide for a Help query, like “To know when the next bus is arriving, just say “open OneBusAway”.

Observed behavior:

Current welcome message is read to the user that ends with "Great! I'm now ready to tell you about arrivals at your stop".

Device:

Amazon Echo.