Add PATH requirement

README

@@ -31,6 +31,7 @@ To be able to auto-build packages, you're going to need:
 
 You then need to create a config.py (copy config.sample.py and follow the
 instructions) to specify the locations of some of these things.
+Also make sure the SDK tools - found in $SDK/tools/ - are in your $PATH.
 
 =MetaData=