Document the use of administrative_email as From:

README.md

@@ -172,6 +172,16 @@ Some bits of configuration are done in `settings.py`: LDAP branches, RSS feeds
 to display on the home page, and so on.  Lastly, it is possible to override
 all Coin templates (i.e. for user views and emails), see below.
 
+Sending emails
+--------------
+
+Coin sends mails, you might want to customize the sender address:
+
+- `DEFAULT_FROM_EMAIL` app-wide setting
+- *administrative email* field, from *ISPDatabase* app, if set, will take
+  precedence over `DEFAULT_FROM_EMAIL` for administrative emails (welcome email
+  and membership fees), will take precedence (if filled).
+
 Cron tasks
 ----------