docs(CHANGES): Note libtmux update

Author: tony

CHANGES

@@ -19,6 +19,13 @@ $ pipx install --suffix=@next 'tmuxp' --pip-args '\--pre' --force
 
 <!-- Maintainers, insert changes / features for the next release here -->
 
+### Internal improvements
+
+- libtmux 0.19.1 -> 0.20.0 (#863)
+
+  Improves support around `Server.new_session` and its params, e.g. `-x` and
+  `-y`.
+
 ## tmuxp 1.25.0 (2023-01-07)
 
 ### Internal improvements