Add note to explain absence of py24 in envlist

tox.ini

@@ -1,5 +1,8 @@
 # !!! When making changes, make sure to also edit .travis.yml !!!
 
+# Note: Supervisor supports Python 2.4 but "py24" is not listed in envlist
+# because tox has a dependency on distribute, which no longer supports 2.4.
+
 [tox]
 envlist =
     py25,py26,py27