[IMP] developer: added "--shell-file" CLI option

Added the description for the new '--shell-file' option, which overrides
the the $PYTHONSTARTUP env variable to initialize the shell session with
a startup Python script.

Related docs: https://docs.python.org/3/using/cmdline.html#envvar-PYTHONSTARTUP

Community PR: odoo/odoo#185075
task-4306704

content/developer/reference/cli.rst

@@ -695,6 +695,11 @@ interaction with the :ref:`orm <reference/orm>` and its functionalities.
       By default, the shell is running in transaction mode. This means that any change made to the
       database is rolled back when exiting the shell. To commit changes, use `env.cr.commit()`.
 
+
+.. option:: --shell-file <init_script.py>
+   Specify a python script to be run after the start of the shell.
+   Overrides the environment variable PYTHONSTARTUP.
+
 .. option:: --shell-interface (ipython|ptpython|bpython|python)
 
    Specify a preferred REPL to use in shell mode. This shell is started with the `env` variable