added a note about the snapshot data migration in 0.7.0

Author: yvess

RELEASE.md

@@ -11,7 +11,7 @@
 - Added site config for homepage_snippet and search_enabled
 
 *Upgrade notes*:
-- Upgrading to 0.7.0 changes to how snapshots are stored and hence requires a snapshot data migration
+- Upgrading to 0.7.0 changes how snapshots are stored. The snapshot data is moved from a postgres json field to an external file. The migration is a normal django data migration and runs in the django database migration process (./manage.py migrate). The snapshot data json field will be removed in a future a release. 
 - For local development symlinking the current nginx configuration is recommended (see README.md for details)
 
 ### 0.6.2