slack import: Document how to send password resets to all users.

This is likely to be an important follow-up step after one finishes
the Slack import.

templates/zerver/help/import-data-from-slack.md

@@ -55,6 +55,29 @@ cd /home/zulip/deployments/current
 ./manage.py import --import-into-nonempty <subdomain> converted_slack_data
 ```
 
+## Logging in
+
+Once the import completes, all your users will have accounts in your
+new Zulip organization, but those accounts won't have passwords yet
+(since for very good security reasons, passwords are not exported).
+Your users will need to either authenticate using something like
+Google auth, or start by resetting their passwords.
+
+You can use the `./manage.py send_password_reset_email` command to
+send password reset emails to your users.  We
+recommend starting with sending one to yourself for testing:
+
+```
+./manage.py send_password_reset_email -u username@example.com
+```
+
+and then once you're ready, you can email them to everyone using e.g.
+```
+./manage.py send_password_reset_email -r '' --all-users
+```
+
+(replace `''` with your subdomain if you're using one).
+
 ## Caveats
 
 - Slack doesn't export private channels or direct messages unless you pay