Remove old notice in README file.

Author: jonathanslenders

README.rst

@@ -10,14 +10,6 @@ Python Prompt Toolkit
 Read the `documentation on readthedocs
 <http://python-prompt-toolkit.readthedocs.io/en/stable/>`_.
 
-NOTICE: prompt_toolkit 3.0
-**************************
-
-Please notice that this branch is the ``prompt_toolkit`` **3.0** branch. For most
-users, it should be compatible with ``prompt_toolkit`` **2.0**, but it requires at
-least **Python 3.6**. On the plus side, ``prompt_toolkit`` **3.0** is completely type
-annotated and uses asyncio natively.
-
 
 Gallery
 *******