Provide a Skill development guide

[ryanisgrig]

Provide a Skill development guide to developers highlighting:

• Take advantage of a ConfirmPrompt/ChoicePrompt rather than a LUIS model for "yes/no", "1/2/first/second/etc." options
• Consider all of the canvases a skill may be used on, but focus on voice as the most important, use adaptive cards to supplement, etc.
• Use Microsoft.Bot.Solutions methods for common scenarios
• Designing a successful language model (LUIS best practices)
• Handling and logging exceptions
• Using the general LUIS model
• Speak variants
• Multiple response options for variation
• Perform async/long running tasks (link to proactive docs)

[ryanisgrig]

@darrenj I think this issue is different than skill development docs related to the new Skills architecture, do we want to split into multiple issues?

https://github.com/Microsoft/AI/blob/master/docs/skills/csharp/developing-a-new-skill.md

[ryanisgrig]

Closing as new doc notes are on #1102