added a warning about 16MB limit

BUGS

@@ -28,4 +28,9 @@ is still possible, for example, to encode a wrapper structure using definite
 length, and encode its substructure member using indefinite length. The BER
 decoder is perfectly capable of decoding such sequences.
 
+6. Encoding or decoding of structures with combined length greater than 16MB
+is not tested. There are some size determination bugs lurking in the produced
+code, which may cause integer overflow in certain edge cases. This restriction
+should not cause a problem in the real world, as higher level protocol typically
+have similar or tighter restrictions on the supported PDU size.