Document the migration process for Slack bots.

[rishig]

Many people considering moving from Slack to Zulip already have a set of custom bots they've written for Slack, and this can be a major blocker in moving them to Zulip. A migration guide would go a long way here.

[kyleboddy]

We are considering the move to Zulip. Is there a guide for this yet? I realize it's four years old, but data migration, while important, is less important than bot transferring and API similarity for our developers.