Added information where to get session ID and secure session ID fixing #128.

doc/REST-API.txt

@@ -10,6 +10,12 @@ The REST API does always return valid JSON data. This includes the non 200
 status responses. If non JSON is received this is an error not generated by the
 API or there was a problem while JSON encoding.
 
+Some end points of this API require a existing session from the channeling
+API, incuding the private secret session ID. To get these it is sufficient
+to connect to the channeling API with a websocket connection. The server
+sends the session ID (Id) and secure session ID (Sid) within the Self 
+document after connection was established.
+
 
 Available end points with request methods and content-type:
 
@@ -153,9 +159,9 @@ Available end points with request methods and content-type:
     This endpoint provides application with user icons
 
     GET application/x-www-form-urlencoded
-      details: 
+      details:
         flags: at the moment the only known flag is s{icon_size_in_pixels},
-               the icons are always squares so only one dimension is needed 
+               the icons are always squares so only one dimension is needed
                example: s46
         imageid and idx: image id received via channeling API (please refer to channeling API for details)
       example: https://example.com/static/img/buddy/s46/8nG33oDk8Yv8fvK6IphL/6vjI2NLigcET/picture.jpg
@@ -172,4 +178,4 @@ https://github.com/strukturag/spreed-webrtc
 For questions, contact mailto:opensource@struktur.de.
 
 
-(c)2014 struktur AG
+(c)2014 struktur AG