update develop installation instructions

DEVELOP.rst

-As :pep:517 does not provide a way to install for development purpose, it is necessary to follow these steps to install for development :
+Installing Odoo Scripts for development
+---------------------------------------
+
+:pep:660 defines `Editable installs for pyproject.toml based builds`, this should not be necessary if your pip version is recent enough.
+
+This should be enough to install in dev mode:
+
+.. code-block:: SH
+
+   python3 -m pip install --user -e ".[docker,import_sql,conf2reST,source_control]" --upgrade
+
+This can be tried to install in dev mode without using the toml file:
 
 .. code-block:: SH
 
    mv pyproject.toml pyproject.toml\~
@@ -2,6 +13,6 @@
 
 .. code-block:: SH
 
    mv pyproject.toml pyproject.toml\~
-   pip3 install --user -e ".[docker,import_sql,conf2reST,source_control]" --upgrade
+   python3 -m pip install --user -e ".[docker,import_sql,conf2reST,source_control]" --upgrade
    mv pyproject.toml\~ pyproject.toml