docs/ipc: add a warning to use an existing library (Thanks slowpoke)

docs/ipc

@@ -19,6 +19,13 @@ calling +i3 --get-socketpath+.
 All i3 utilities, like +i3-msg+ and +i3-input+ will read the +I3_SOCKET_PATH+
 X11 property, stored on the X11 root window.
 
+[WARNING]
+.Use an existing library!
+There are existing libraries for many languages. You can have a look at
+<<libraries>> or search the web if your language of choice is not mentioned.
+Usually, it is not necessary to implement low-level communication with i3
+directly.
+
 == Establishing a connection
 
 To establish a connection, simply open the IPC socket. The following code
@@ -667,7 +674,9 @@ mode is simply named default.
 { "change": "default" }
 ---------------------------
 
-== See also
+== See also (existing libraries)
+
+[[libraries]]
 
 For some languages, libraries are available (so you don’t have to implement
 all this on your own). This list names some (if you wrote one, please let me