Add comment regarding experimental-use service bits

As per mailing list discussion.

src/protocol.h

@@ -64,6 +64,14 @@ class CMessageHeader
 enum
 {
     NODE_NETWORK = (1 << 0),
+
+    // Bits 24-31 are reserved for temporary experiments. Just pick a bit that
+    // isn't getting used, or one not being used much, and notify the
+    // bitcoin-development mailing list. Remember that service bits are just
+    // unauthenticated advertisements, so your code must be robust against
+    // collisions and other cases where nodes may be advertising a service they
+    // do not actually support. Other service bits should be allocated via the
+    // BIP process.
 };
 
 /** A CService with information about it as peer */