docs(CHANGES): Note the server being connected / disconnected

Author: tony

CHANGES

@@ -14,6 +14,13 @@ $ pip install --user --upgrade --pre libtmux
 
 <!-- Maintainers and contributors: Insert change notes for the next release above -->
 
+### New features
+
+#### Detect if server active (#448)
+
+- `Server.is_alive()`
+- `Server.raise_if_dead()`
+
 ### Internal
 
 - Remove unused `sphinx-click` development dependency