added warning to CHANGELOG about running sk migration while online

CHANGELOG.rst

@@ -8,8 +8,12 @@ the top.  Include a label indicating the component affected.
 **models:** added a new sorting key and index to `Comment` documents, removing the need
 for certain hierarchical db queries.  Also added a copy of the author's username 
 to `Comment` and `CommentThread` models, to reduce the number db queries.  
-IMPORTANT: these changes require a data backpopulation to be run BEFORE deploying 
+IMPORTANT: 
+- these changes require a data backpopulation to be run BEFORE deploying 
 updated code.  The backpopulation script is located at 
 scripts/db/migrate-001-sk-author_username.js 
 and should be run directly against your MongoDB instance.
+- If your application is online while the migration script is running, you should run
+the script a second time after deploying the application.  This is to backpopulate any
+new content that may have been created between the first run and the application update.