README: Remove warning about the project being in beta stage.

It's been over 12 years and the project is relatively stable now.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

README.md

@@ -6,13 +6,10 @@ The MicroPython project
   <img src="https://raw.githubusercontent.com/micropython/micropython/master/logo/upython-with-micro.jpg" alt="MicroPython Logo"/>
 </p>
 
-This is the MicroPython project, which aims to put an implementation
-of Python 3.x on microcontrollers and small embedded systems.
+This is the MicroPython project, an implementation of Python 3.x for
+microcontrollers, embedded systems and other constrained platforms.
 You can find the official website at [micropython.org](http://www.micropython.org).
 
-WARNING: this project is in beta stage and is subject to changes of the
-code-base, including project-wide name changes and API changes.
-
 MicroPython implements the entire Python 3.4 syntax (including exceptions,
 `with`, `yield from`, etc., and additionally `async`/`await` keywords from
 Python 3.5 and some select features from later versions). The following core