add CHANGELOG.rst

CHANGELOG.rst

+Change Log
+----------
+
+These are notable changes in cs_comments_service.  This is a rolling list of changes,
+in roughly chronological order, most recent first.  Add your entries at or near
+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 
+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.
+