bitbake: README: add required python version for bitbake

(Bitbake rev: 8f9402b291421ebcbf0a3ab97791c87af4b3f36e)

Signed-off-by: Frank de Brabander <debrabander@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/README

@@ -13,6 +13,8 @@ Bitbake plain documentation can be found under the doc directory or its integrat
 html version at the Yocto Project website:
     https://docs.yoctoproject.org
 
+Bitbake requires Python version 3.8 or newer.
+
 Contributing
 ------------