chore: add postgres in developer doc (#45)

Author: hadrien

README.md

@@ -398,8 +398,9 @@ It returns the path of  `alembic.ini` configuration file. By default, it returns
 
 ## Prerequisites
 
-- **Python ^3.9**
+- **Python >=3.9**
 - [**Poetry**](https://poetry.eustace.io/) to install package dependencies.
+- A postgres DB reachable at `postgresql://postgres@localhost/postgres`
 
 
 ## Setup