Describe the roster tab

doc/en/usage.txt

@@ -42,6 +42,22 @@ You can go from one tab to another in many ways:
 * Alt+e, this will jump to the next tab with the highest priority. Priority
   applies in this order: private message > highlight message > normal message.
 
+Roster tab
+~~~~~~~~~~
+This is a unique tab, always numbered _0_. It contains the list of your
+contacts. You can add/remove/edit/search contacts from there, and you can open
+a conversation with one of them.
+
+Use the _arrows_ to browse the list, the _space_ key to fold or unfold a group
+or a contact.
+
+image::../images/roster.png["The roster tab", title="The roster tab"]
+
+* _1_: The area where information messages are displayed.
+* _2_: The actual list of contacts. The first level is group, the second is the
+* contacts and the third is the resources of you online contacts.
+* _3_: More informations about the selected contact.
+
 Commands
 --------