Instructions for 'keybase prove twitter' are not complete

The instructions state the following:

`Please publicly tweet the following, and don't delete it:`

`Verifying myself:...`

This didn't work. In order for the verification to work, I needed to [pin the tweet to my public profile](https://twitter.com/support/status/458707240325615617). Once I pinned the tweet to my twitter public profile, my identity was able to be verified.

Start with the instructions for `keybase prove twitter`; the issue identifies the existing tweet-verification wording as incomplete. Update the instructions to explain that the verification tweet must be pinned to the user's public Twitter profile, then verify that the documented steps match the reported successful flow.

Written by the indexing model from the issue text.