Add prelude-python-mode-set-encoding-automatically defcustom (#1293)

- if it is nil (default) does nothing
- if non-nil insert '# coding: utf-8' on top of python buffers

doc/modules/python.md

@@ -20,3 +20,16 @@ syntax checkers, [Pylint](http://www.pylint.org/) and
 order to have Flycheck support on the fly syntax checking for
 Python you need to have either of these installed and accessible to
 Emacs. In order to manually choose a checker run `C-c ! s`.
+
+
+## Automatic insertion of # coding: utf-8
+
+Previously `prelude-python` had this feature enabled by default, but
+that is only necessary on Python2, because Python3 already use utf-8
+as default encoding. In 2020, python2 becames deprecated, so that
+functionallity becames a annoying side-effect for some users. If you
+wish to enable this, add this to your config file:
+
+```emacs-lisp
+(setq prelude-python-mode-set-encoding-automatically t)
+```