readme: specify 'customer invoice'

Following the arrival of supplier invoice templates in Dolibarr 16, it's better
to specify than this module only manage customer invoices.

Adding the management of supplier invoice seems possible but I don't really see
a use case at the moment.

README.md

@@ -2,9 +2,9 @@
 
 ## Features
 
-(en) This module sends by email the invoice generated with recurring invoices via scheduled jobs.
+(en) This module sends by email the customer invoice generated with a recurring invoice template via scheduled jobs.
 
-(fr) Ce module envoie par mail les factures générées automatiquement par les travaux planifiés et les factures modèles.
+(fr) Ce module envoie par mail les factures clientes générées automatiquement par les travaux planifiés et les factures modèles.
 
 You can customize the mail globally or by recurring invoice.