python: Add dbscheme regeneration instructions

Author: igfoo

python/extractor/semmle/dbscheme_gen.py

@@ -62,6 +62,7 @@ def write(nodes, out):
 
 HEADER = '''/*
  *      This dbscheme is auto-generated by '%s'.
+ *      Run "make dbscheme" in python/extractor/ to regenerate.
  *      WARNING: Any modifications to this file will be lost.
  *      Relations can be changed by modifying master.py or
  *      by adding rules to dbscheme.template