README.md: Update subdir descriptions.

README.md

@@ -25,20 +25,24 @@ See the repository www.github.com/micropython/pyboard for the Micro
 Python board, the officially supported reference electronic circuit board.
 
 Major components in this repository:
-- py/ -- the core Python implementation, including compiler and runtime.
+- py/ -- the core Python implementation, including compiler, runtime, and
+  core library.
 - unix/ -- a version of Micro Python that runs on Unix.
 - stmhal/ -- a version of Micro Python that runs on the Micro Python board
   with an STM32F405RG (using ST's Cube HAL drivers).
-- teensy/ -- a version of Micro Python that runs on the Teensy 3.1
-  (preliminary but functional).
+- minimal/ -- a minimal Micro Python port. Start with this if you want
+  to port Micro Python to another microcontroller.
 
 Additional components:
-- bare-arm/ -- a bare minimum version of Micro Python for ARM MCUs.  Start
-  with this if you want to port Micro Python to another microcontroller.
+- bare-arm/ -- a bare minimum version of Micro Python for ARM MCUs. Used
+  mostly to control code size.
+- teensy/ -- a version of Micro Python that runs on the Teensy 3.1
+  (preliminary but functional).
 - unix-cpy/ -- a version of Micro Python that outputs bytecode (for testing).
 - tests/ -- test framework and test scripts.
 - tools/ -- various tools, including the pyboard.py module.
 - examples/ -- a few example Python scripts.
+- docs/ -- official documentation in RST format.
 
 "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).