Update changelog. Bump version.

Author: brocaar

docs/config.toml

@@ -55,5 +55,5 @@ theme = "chirpstack-hugo-theme"
     weight = 7
 
 [params]
-version = "3.8.1"
+version = "3.9.0"
 component = "chirpstack-network-server"

docs/content/overview/changelog.md

@@ -9,6 +9,23 @@ description: Lists the changes per ChirpStack Network Server release, including
 ---
 # Changelog
 
+## v3.9.0
+
+### Features
+
+#### Redis Cluster and Sentinel
+
+This release introduces the support for [Redis Cluster](https://redis.io/topics/cluster-tutorial)
+and [Redis Sentinel](https://redis.io/topics/sentinel). 
+
+#### Rejected uplink callback to Network Controller
+
+A new API method has been added to the (optional) Network Controller called
+`HandleRejectedUplinkFrameSet`. When ChirpStack Network Server rejects an
+uplink (e.g. when the device is not known to the network, or the activation
+is missing), it will call this method (when the Network Controller is
+configured).
+
 ## v3.8.1
 
 ### Bugfixes