refer to an issue in ghostferry instead of an internal one

docs/source/copydbforeignkey.rst

@@ -4,7 +4,10 @@
 Running ``ghostferry-copydb`` in production for tables with Foreign Key Constraints
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-Migrating tables with foreign keys constraints is an experimental feature in copydb and should be used at your own risk in production.
+Migrating tables with foreign keys constraints is an experimental feature in copydb and should be used at your own risk in production. Currently migrating tables with foreign key constraints is only possible
+if the source database is in read_only mode. Since tables with foreign key constraints can have referential actions for a foreign key such as ``ON DELETE CASCADE``, ``ON UPDATE CASCADE``. Cascading deletes and updates
+in child tablees caused by foreign key constraints don't show up in binlogs because these referential actions are dealt internally by InnoDB. This issue descibes briefly why the source database should be 
+read_only during the migration - https://github.com/Shopify/ghostferry/issues/289.
 
 
 Prerequisites