update iobber readme

README

@@ -41,3 +41,8 @@ NOTE: the current version of iobber does not recognise chunks' syntactic heads.
 Also, it is unable to recognise discontinuous chunks. Both types of information
 may be included in ccl format and this is likely to be supported in the future.
 
+
+If the input file contains annotations in channels other than those defined
+in the config, they will be preserved (this makes it possible to e.g. chunk
+files already annotated with named entities).
+