Serialisation: add reference to MessagePack.

README.md

@@ -22,7 +22,7 @@ and modules which are documented and supported.
   2.4 [DS3231 precision RTC](./README.md#24-ds3231-precision-rtc) Use cheap hardware to calibrate Pyboard RTC.  
  3. [Essays](./README.md#3-essays) General thoughts.  
   3.1 [Resilient](./README.md#31-resilient) A guide to writing resilient WiFi code  
-  3.2 [Serialisation](./README.md#32-serialisation) Review of MicroPython's four serialisation libraries  
+  3.2 [Serialisation](./README.md#32-serialisation) Review of MicroPython's five serialisation libraries  
   3.3 [Measurement of relative timing and phase of fast analog signals](./README.md#33-measurement-of-relative-timing-and-phase-of-fast-analog-signals) For Pyboard.  
  4. [Code samples](./README.md#4-code-samples) Samples prefixed Pyboard are Pyboard specific  
   4.1 [Pyboard Mutex](./README.md#41-pyboard-mutex) Share data between threads and ISR's.  
@@ -139,7 +139,7 @@ Probably applies to other WiFi connected MicroPython devices.
 ## 3.2 Serialisation
 
 [A discussion](./SERIALISATION.md) of the need for serialisation and of the
-relative characteristics of four libraries available to MicroPython. Includes a
+relative characteristics of five libraries available to MicroPython. Includes a
 tutorial on a Protocol Buffer library.
 
 ## 3.3 Measurement of relative timing and phase of fast analog signals

SERIALISATION.md

@@ -40,10 +40,11 @@ the byte sequences are relatively long.
 Numbers 1, 2 and 4 are self-describing: the format includes a definition of its
 structure. This means that the decoding process can re-create the object in the
 absence of information on its structure, which may therefore change at runtime.
-Self describing formats inevitably are variable length. This means that the
-receiving process must be provided with a means to determine when a complete
-message has been received. In the case of ASCII formats a delimiter may be used
-but in the case of `MessagePack` this presents something of a challenge.
+Self describing formats inevitably are variable length. This is no problem
+where data is being saved to file, but if it is being communicated across a
+link the receiving process needs a means to determine when a complete message
+has been received. In the case of ASCII formats a delimiter may be used but in
+the case of `MessagePack` this presents something of a challenge.
 
 The `ustruct` format is binary: the byte sequence comprises binary data which
 is neither human readable nor self-describing. The problem of message framing