Add documentation about SUBSCRIPTION_REFERENCE setting.

README.md

@@ -277,6 +277,7 @@ List of available settings in your `settings_local.py` file.
 - `EXTRA_TEMPLATE_DIRS`: See *Customizing templates*
 - `LDAP_ACTIVATE`: See *LDAP*
 - `MEMBER_MEMBERSHIP_INFO_URL`: Link to a page with information on how to become a member or pay the membership fee
+- `SUBSCRIPTION_REFERENCE`: Pattern used to display a unique reference for any subsciption. Helpful for bank wire transfer identification
 
 More information
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