Document toll-free option

[damiendaemon]

Done in this PR:

• It is possible to get a dedicated phone number without signing up for a 10DLC. We document this process in Pinpoint notebook and readme.
• There are some changes to the UI for Pinpoint signing up for long code. We merge this into documentation.
• We add instructions for setting up long code to demo guide.

Next steps would be to:

• Use Pinpoint Auto Workshop to attach phone number entered into CloudFormation parameters with TextAlerts SNS topic and ensure two-way SMS is set up.
• Rename "long code" parameters to something more like "dedicated phone number".

Issue #207 and #244 and others seem to be looking for a way around having to register a company etc. and that whole involved process related to 10DLC so we should be able to point at these instructions.

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This work was done by the Dae.mn Team. http://dae.mn/ml