Define what a sprinkles requires bare minimum

[Silic0nS0ldier]

There was a misunderstanding in the support channel related to this, as it was assumed a sprinkle required some form of UI contribution.

... add that to the docs. Because everything I read implied I needed more than that.  Even on the introduction page, adding 'A valid sprinkle can be an empty directory' would have been enough

[lcharette]

Related :

• Factor our core, account and admin as independent sprinkles UserFrosting#830 (comment)

  A default install should also include a site sprinkle containing the default homepage. This would be intended to be the base for the first sprinkle a new user create.

• CLI command to create a sprinkle UserFrosting#729

[lcharette]

We should also add a warning block somewhere to further clarify the "don't edit anything in the core sprinkle" concept.