[IMP] doc: improve guidelines related to Git and commit messages
[IMP] doc: improve guidelines related to Git and commit messages

Purpose of this commit is to explain with more details how to write commit
messages. Content of the Git section is also updated with last decisions
and conventions.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo