Lack of documentation for Burner

[flow-slack-issue-bot]

Reported by: Brian Doyle (<@brian.doyle>)

Issue Description:

Someone want to know how to emit a Destroy event for Flow NFT, but there is none in our example code.
@joshua

@brian.doyle
I had to introduce flow-ft and the burner contract, and explain how to add a burner to Flow NFT
I searched in our documents, and there is no documented introduction to Burner and its related mechanisms.