Add CTA to Slack import doc (#6888)

* Add CTA to Slack import doc

source/onboard/migrate-from-slack.rst

@@ -4,6 +4,10 @@ Migrate from Slack
 .. include:: ../_static/badges/allplans-selfhosted.rst
   :start-after: :nosearch:
 
+.. note::
+
+  `Slack's data control policies may change at any time <https://twitter.com/Austen/status/1752064934970896626>`_. Also, Slack may choose to changes its export capabilities, and/or charge fees to customers for exporting data stored in Slack. Support for negotiating export of customer IP from Slack Enterprise can be requested by `contacting Mattermost sales team <https://mattermost.com/contact-sales/>`_.
+
 Migrating from Slack to Mattermost involves the following steps:
 
 1. `Prepare your Mattermost server <#prepare-your-mattermost-server>`__