Update changelog for v1.6.0 release (#6009)

## Motivation

This PR updates the changelog for the `v1.6.0` release, which will be released as beta after merging and if no issues are found during testing released as stable.

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 See [RELEASE](./RELEASE.md) for workflow instructions.
 
-## UNRELEASED
+## Release v1.6.0
 
 ### Upgrade information
 
@@ -17,6 +17,28 @@ Upgrading to this version requires going through v1.5.x first. Removed migration
 
 ### Highlights
 
+This update introduces certificate authentication against PoETs. The default values are included in the mainnet preset
+and changing them is not needed unless you experience connection issues to a certifier:
+
+```json
+  "main": {
+    "certifier": {
+      "client": {
+        "retry-delay": "1s",
+        "max-retry-delay": "30s",
+        "max-retries": 5,
+      }
+    }
+  }
+```
+
+The usage of a certifier needs to be supported by the PoET your node is connecting to. If you are using the default
+PoETs the switch will happen automatically in the near future, so no action is needed from your side. If you are
+operating your own PoET and want to use certificate authentication please refer to the documentation:
+
+* [PoET README](https://github.com/spacemeshos/poet/blob/main/README.md) for configuration of the PoET.
+* [certifier README](https://github.com/spacemeshos/post-rs/blob/main/certifier/README.md) for setting up a certifier.
+
 ### Features
 
 ### Improvements

README.md

@@ -432,7 +432,8 @@ are covered by tests and which not.
 
 ### Continuous Integration
 
-We've enabled continuous integration on this repository in GitHub. You can see more details about our CI workflows on the [Actions tab](https://github.com/spacemeshos/go-spacemesh/actions).
+We've enabled continuous integration on this repository in GitHub. You can see more details about our CI workflows on
+the [Actions tab](https://github.com/spacemeshos/go-spacemesh/actions).
 
 ### Docker
 

config/mainnet.go

@@ -203,5 +203,6 @@ func MainnetConfig() Config {
 			RetryInterval: time.Minute,
 			Tries:         20,
 		},
+		Certifier: activation.DefaultCertifierConfig(),
 	}
 }