document friends mobile API

doc/mobile/friends.md

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+# Online friends
+
+This section requires authentication.
+
+## Get list of online friends
+
+Request the list of your online friends from any authenticated websocket connection:
+
+```javascript
+// send
+{t: 'following_onlines'}
+```
+
+The server answers with:
+
+```javascript
+// receive
+{t: "following_onlines", d: ["claymore", "TrialB", "FM amazingoid"]}
+```
+
+Note that the full player name is provided, title included.
+This allows direct display but requires some gymnastic to infer the user ID:
+
+```javascript
+function userFullNameToId(fullName) {
+    var split = fullName.split(' ');
+    var id = split.length == 1 ? split[0] : split[1];
+    return id.toLowerCase();
+}
+```
+
+## Be notified of friends entering and leaving
+
+The server continually sends these events through any authenticated websocket connection:
+
+```javascript
+// receive
+{t: "following_enters", d: "Hasimir"}
+```
+
+```javascript
+// receive
+{t: "following_leaves", d: "claymore"}
+```
+
+Note that the data is a string, not an array.