précise des détails dans le readme

README.md

@@ -5,6 +5,7 @@ Simple mailing script.
 **Table of content**
 
 - [Installation](#installation)
+- [Input format](#input-format)
 - [Licence](#licence)
 
 ## Installation
@@ -29,6 +30,19 @@ following packages:
         $ ./mailing.py --help
         $ ./mailing.py -t test-recipients-data -b test-emailling --dry-run --verbose
 
+## Input format
+
+The « to-file » is expected to be csv with the [default
+dialect](https://docs.python.org/3/library/csv.html?highlight=csv#dialects-and-formatting-parameters)
+(delimiter = `,` and char delimiter =`"`).
+
+The first line declare columns headers. The `from` and `to` headers are
+expected the be found.
+
+To avoid boring repetition it is possible to declare a default column value
+using `:` in header. The default value is considered when the column is lacking,
+not when the column is empty.
+
 ## License
 
 Spamotron is developed by April and licensed under the [GPLv2+](LICENSE).