Added changelog

CHANGELOG.md

+# Change Log
+All notable changes to this project will be documented in this file.
+
+## 2015-10-06
+### Added
+- banner warning if the user is looking at an lvem_view page (#2600)
+- this changelog file (#2599)
+- added event page link to labelling email alerts (#2575)
+- add value from coinc_event.likelihood column to "Coinc Tables" for gstlal
+  events (#2513)
+
+### Changed
+- interpretation of values for cWB events (see #2484)
+- list of labels for query now generated by DB query instead of static list
+  (#2523)
+
+### Fixed
+- remove user from groups that are not present in IdP shibboleth assertion
+  (#2600)