README: Add short overview of Python features supported.

Also, "upgrade" project to "early beta", and elaborate pyboard description.

README.md

@@ -11,12 +11,19 @@ The Micro Python project
 This is the Micro Python project, which aims to put an implementation
 of Python 3.x on a microcontroller.
 
-WARNING: this project is in its early stages and is subject to large
+WARNING: this project is in early beta stage and is subject to large
 changes of the code-base, including project-wide name changes and API
 changes.
 
+Micro Python implements entire Python 3.4 syntax (including exceptions,
+"with", "yield from", etc.). Following core datatypes are provided:
+str (no Unicode support yet), bytes, bytearray, tuple, list, dict, set,
+array.array, collections.namedtuple, classes and instances. Builtin
+modules include sys, time, and struct. Note that only subset of
+Python 3.4 functionality implemented for the data types and modules.
+
 See the repository www.github.com/micropython/pyboard for the Micro
-Python board.
+Python board, officially supported reference electronic circuit board.
 
 Major components in this repository:
 - py/ -- the core Python implementation, including compiler and runtime.