Update CHANGELOG for Mimir CSS

Was so excited about merging that I forgot to update the CHANGELOG.md

Ref: #1482

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

CHANGELOG.md

@@ -5,6 +5,7 @@
 * [CHANGE] Compactor: No longer upload debug meta files to object storage. #1257
 * [FEATURE] Distributor: Added the ability to forward specifics metrics to alternative remote_write API endpoints. #1052
 * [ENHANCEMENT] Ruler: Add more detailed query information to ruler query stats logging. #1411
+* [ENHANCEMENT] Admin: Admin API now has some styling. #1482
 * [BUGFIX] Fix internal modules to wait for other modules depending on them before stopping. #1472
 
 ### Mixin