Make it clear that it's allowed to prefix the verification tweet with '@...', so it won't show up in a user's regular timeline

I didn't want to bother my followers with a meaningless technical tweet, so I tried prefixing it with `@KeybaseIO` so it would only appear on my "tweets & replies" tab. This format turned out to be accepted by the verification mechanism.

I think this format should be the default, as it's more discrete.
Otherwise, I think it should be stated that one is allowed to do this.

I think it's a great thing that this format is accepted by Keybase, and I think it should be documented.

Issue #1044 describes an accepted verification-tweet format that prefixes the tweet with @KeybaseIO, but it names no documentation file or test. Find the relevant Keybase verification instructions first, then document that this prefix is allowed; done means users can discover the option and understand its timeline behavior.

Written by the indexing model from the issue text.