[IMP] doc: add guidelines warning
[IMP] doc: add guidelines warning

To prevent huge chunk of modifications for styling reasons (e.g. mass PEP8
commits)

ygol/odoo,Elico-Corp/odoo_OCB,microcom/odoo,dfang/odoo,dfang/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,dfang/odoo,storm-computers/odoo,hip-odoo/odoo,laslabs/odoo,microcom/odoo,storm-computers/odoo,microcom/odoo,hip-odoo/odoo,laslabs/odoo,microcom/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,microcom/odoo,storm-computers/odoo,bplancher/odoo,sysadminmatmoz/OCB,ygol/odoo,hip-odoo/odoo,Elico-Corp/odoo_OCB,sysadminmatmoz/OCB,dfang/odoo,hip-odoo/odoo,sysadminmatmoz/OCB,sysadminmatmoz/OCB,storm-computers/odoo,microcom/odoo,ygol/odoo,sysadminmatmoz/OCB,ygol/odoo,bplancher/odoo,ygol/odoo,hip-odoo/odoo,sysadminmatmoz/OCB,bplancher/odoo,storm-computers/odoo,hip-odoo/odoo,bplancher/odoo,bplancher/odoo,dfang/odoo,sysadminmatmoz/OCB,laslabs/odoo,laslabs/odoo,ygol/odoo,ygol/odoo,Elico-Corp/odoo_OCB,dfang/odoo,storm-computers/odoo,Elico-Corp/odoo_OCB