Update readme and decomission wiki

[garypretty]

Related to microsoft/botframework-sdk#5898

The readme for the .NET repository needs to be updated as part of a broader review across the languages and in order to decommission the wiki's for each repository.

• Update introduction text to remove obsolete links (e.g. old botframework repo)
• Move build instructions from wiki to readme
• Update contributing section to link to contributing guidelines (avoid duplication)
• Move license information from wiki to readme
• Consolidate feedback / support channels information from wiki with existing info in readme

[garypretty]

Wiki also now hidden.