Update CHANGELOG.md

CHANGELOG.md

@@ -9,6 +9,7 @@ FEATURES:
 IMPROVEMENTS:
 
 * agent: Switched to using a read lock for the agent's RPC dispatcher, which prevents RPC calls from getting serialized. [GH-3376]
+* agent: When joining a cluster, Consul now skips the unique node ID constraint for Consul members running Consul older than 0.8.5. This makes it easier to upgrade to newer versions of Consul in an existing cluster with non-unique node IDs. [GH-3070]
 * build: Upgraded Go version to 1.9. [GH-3428]
 * server: Consul servers can re-establish quorum after all of them change their IP addresses upon a restart. [GH-1580]