Actually add detailed information about the workaround

Author: ronaldoussoren

docs/notes/using-nsxpcinterface.rst

@@ -0,0 +1,32 @@
+Using NSXPCInterface from Python
+================================
+
+The Foundation class ``NSXPCInterface`` requires an Objective-C
+protocol to define the API of the interface. Those protocols
+cannot be defined in Python using :class:`objc.formal_protocol`
+because the Cocoa class requires some data ("extended method sigantures")
+that cannot be registered using the public API for the Objectie-C
+runtime.
+
+If you do try to use a protocol defined in Python with ``NSXPCInterface``
+you'll get an error similar to this:
+
+::
+       NSInvalidArgumentException - NSXPCInterface: Unable to get extended method signature from Protocol data (MyProtocol / runCommand:withReply:). Use of clang is required for NSXPCInterface.
+
+
+This means that any custom protocols that will be used
+with ``NSXPCInterface`` need to be defined in a C extension which
+is compiled using the clang compiler (the compiler used by Xcode).
+
+The compiler will elide protocol information from the binary for
+all protocols that aren't actually used. To ensure that the protocol
+information is included add an (unused) function that appears to
+use the protocol, for example:
+
+.. sourcecode:: Objective-C
+
+   static void use_protocol(void) __attribute__((__unused__))
+   {
+       printf("%p\n", @protocol(MyProtocol));
+   }