docs: add README blurb pointing to psycopg 3 on PyPI

See #1632.

Author: dvarrazzo

README.rst

@@ -17,6 +17,18 @@ flexible objects adaptation system.
 
 Psycopg 2 is both Unicode and Python 3 friendly.
 
+.. Note::
+
+    The psycopg2 package is still widely used and actively maintained, but it
+    is not expected to receive new features.
+
+    `Psycopg 3`__ is the evolution of psycopg2 and is where `new features are
+    being developed`__: if you are starting a new project you should probably
+    start from 3!
+
+    .. __: https://pypi.org/project/psycopg/
+    .. __: https://www.psycopg.org/psycopg3/docs/index.html
+
 
 Documentation
 -------------