Mention fork in changelog

CHANGELOG.md

@@ -1,6 +1,19 @@
 Sydent 2.6.0 (2023-11-20)
 =========================
 
+Sydent will soon be forked by Element under an AGPLv3.0 licence (with CLA, for
+proprietary dual licensing). We expect this to be the last release of Sydent
+under the current Apache 2 licence.
+
+You can read more about this here:
+
+- https://matrix.org/blog/2023/11/06/future-of-synapse-dendrite/
+- https://element.io/blog/element-to-adopt-agplv3/
+
+The Matrix.org Foundation copy of the project will be archived. Any changes needed
+by server administrators will be communicated in Sydent's changelog, but we are
+striving to make this as seamless as possible.
+
 Features
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