Added missing changelog entry for new guide added for Multiple Databases with...

Added missing changelog entry for new guide added for Multiple Databases with Active Record(#36389) [ci skip]

guides/CHANGELOG.md

 ## Rails 6.0.0.rc2 (July 22, 2019) ##
 
-*   No changes.
+*   Add "Multiple Databases with Active Record" Guide.
+
+    *Eileen M. Uchitelle*
 
 
 ## Rails 6.0.0.rc1 (April 24, 2019) ##