Add an help text about the 'validated' field of invoices

coin/billing/models.py

@@ -41,7 +41,9 @@ class Invoice(models.Model):
         ('trouble', 'Litige')
     )
 
-    validated = models.BooleanField(default=False, verbose_name='validée')
+    validated = models.BooleanField(default=False, verbose_name='validée',
+                                    help_text='Once validated, a PDF is generated'
+                                    ' and the invoice cannot be modified')
     number = models.CharField(max_length=25,
                               default=next_invoice_number,
                               unique=True,