sys.path when interfaces.py is invoked

per: https://sourceforge.net/p/roundup/mailman/message/26389589/
Document that tracker lib and other tracker specific module dirs are
not on the sys.path when interfaces.py is invoked.

Author: rouilj

doc/customizing.txt

@@ -1411,8 +1411,11 @@ but with great power comes great responsibility.
 
 Interfaces.py has been around since the earliest releases of roundup
 and used to be the main way to get a lot of customization done. In
-modern roundup, the extensions_ mechanism is used, but there are places
-where interfaces.py is still useful.
+modern roundup, the extensions_ mechanism is used, but there are
+places where interfaces.py is still useful. Note that the tracker
+directories are not on the Python system path when interfaces.py is
+evaluated. You need to add library directories explicty by
+modifying sys.path.
 
 Example: Changing Cache-Control headers
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