added mailbox wiki link

README.md

@@ -80,10 +80,12 @@ any hardware supporting Java (e.g. unix server, raspberry pi) could be added.
 * Push-like message notification for the owner to decrease battery drain
 
 ## Useful links 
-[briarproject.org](https://briarproject.org/) 
-
 [Source code](https://code.briarproject.org/briar/briar-mailbox/-/tree/main) 
 
+[Mailbox Architecture](https://code.briarproject.org/briar/briar/-/wikis/Mailbox-Architecture)
+
+[briarproject.org](https://briarproject.org/)
+
 
 ## Donate 
 [![Donate using Liberapay](https://briarproject.org/img/liberapay.svg)](https://liberapay.com/Briar/donate) [![Flattr this](https://briarproject.org/img/flattr-badge-large.png "Flattr this")](https://flattr.com/t/592836/)