Add warning for old lamp stack.

Support of lampconfigure should be added in new stack, then this one can be safely dropped

stack/lamp.cfg

+# Warning: this file is deprecated and is here only for compatiblity.
+# If you extend this file, you should probably extend stack/lamp/buildout.cfg
+# Instead.
+
+# Only reason to use this file is when your PHP application uses "delete",
+# "rename" or any feature of lampconfigure (not yet ported to new stack).
+
 [buildout]
 parts =
   template