Create a recipe on how to use settings.

[isaacabraham]

The recipe should link to the official ASP .NET docs, as well as:

• Example use of appsettings.json (with big fat warnings about not committing secrets into plain text files)
• Use of IConfiguration to retrieve settings.
• Important note about Saturn overriding use of settings providers vis-a-vis User Secrets.

Note that this recipe should be about how to add settings as quickly and easily as possible - NOT a guide on the pros and cons of different settings providers etc.