README: Improve description of precompiled bytecode; mention mpy-cross.

README.md

@@ -22,8 +22,9 @@ Builtin modules include `sys`, `time`, and `struct`, etc. Select ports have
 support for `_thread` module (multithreading). Note that only a subset of
 Python 3 functionality is implemented for the data types and modules.
 
-MicroPython can execute scripts in source form or precompiled to bytecode,
-either from on-device filesystem or "frozen" into MicroPython executable.
+MicroPython can execute scripts in textual source form or from precompiled
+bytecode, in both cases either from an on-device filesystem or "frozen" into
+the MicroPython executable.
 
 See the repository http://github.com/micropython/pyboard for the MicroPython
 board (PyBoard), the officially supported reference electronic circuit board.
@@ -31,6 +32,8 @@ board (PyBoard), the officially supported reference electronic circuit board.
 Major components in this repository:
 - py/ -- the core Python implementation, including compiler, runtime, and
   core library.
+- mpy-cross/ -- the MicroPython cross-compiler which is used to turn scripts
+  into precompiled bytecode.
 - unix/ -- a version of MicroPython that runs on Unix.
 - stmhal/ -- a version of MicroPython that runs on the PyBoard and similar
   STM32 boards (using ST's Cube HAL drivers).