README.md

@@ -146,3 +146,11 @@ PROVISIONING_DURATIONS += [
     (datetime.timedelta(days=365*8), '8 ans'),
 ]
 ```
+
+**Warning:** please note that the first element of each
+`RESOURCES_UNITS` tuple will be stored in the database as a string,
+which length must be less or equal to **10**.
+
+You must note too, that the first element of each
+`PROVISIONING_DURATIONS` tuple should be a `datetime.timedelta` object
+or, at least, something having a `days` attribute, storing an integer.