Add instructions for publishing to feeds other than sbot's current master

[Paul Frazee]

Normally you call publish() to create new messages, and that uses the master feed in the sbot instance.
If you want to use a different feed, you open a feed instance in your program, generate the message, and use sbot's add().

This needs to be documented.

[Dominic Tarr]

👍

[stale]

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