README: Add explicit note that subdirs contain more READMEs.

README.md

@@ -54,6 +54,8 @@ Additional components:
 - tools/ -- various tools, including the pyboard.py module.
 - examples/ -- a few example Python scripts.
 
+The subdirectories above may include READMEs with additional info.
+
 "make" is used to build the components, or "gmake" on BSD-based systems.
 You will also need bash and Python (at least 2.7 or 3.3).