featureset version in RN5-6-0 (RedisLabs#759)

Author: AlonMagrafta

content/rs/release-notes/rs-5-6-0-april-2020.md

@@ -21,6 +21,7 @@ categories: ["RS"]
 ### Upgrade
 
 - Follow [these instructions]({{< relref "/rs/installing-upgrading/upgrading.md" >}}) for upgrading to RS 5.6.0 from RS 5.0.2 and above.
+- For Active-Active databases, you must upgrade the [feature set version]({{< relref "/rs/installing-upgrading/upgrading.md#upgrading-active-active-databases" >}}).
 
 ### End of Life