README.md: Add note about callback execution speed.

README.md

@@ -155,7 +155,7 @@ under development so check for updates.
 9. [Realtime applications](./README.md#9-realtime-applications) Accommodating tasks requiring fast RT performance.  
 10. [ePaper displays](./README.md#10-epaper-displays) Guidance on using ePaper displays.  
 
-[Appendix 1 Application design](./README.md#appendix-1-application-design) Tab order, button layout, encoder interface, use of graphics primitives  
+[Appendix 1 Application design](./README.md#appendix-1-application-design) Tab order, button layout, encoder interface, use of graphics primitives, more on callbacks.
 [Appendix 2 Freezing bytecode](./README.md#appendix-2-freezing-bytecode) Optional way to save RAM.  
 [Appendix 3 Cross compiling](./README.md#appendix-3-cross-compiling) Another way to save RAM.  
 
@@ -809,6 +809,10 @@ are passed, bearing in mind the first arg described above. An incorrect
 argument count results in puzzling tracebacks which appear to implicate the GUI
 code. This is because it is the GUI which actually executes the callbacks.
 
+Callbacks should complete quickly. See
+[Appendix 1 Application design](./README.md#appendix-1-application-design) for
+discussion of this.
+
 ###### [Contents](./README.md#0-contents)
 
 ## 2.3 Colors
@@ -3339,6 +3343,31 @@ micro-gui.
 
 The `primitives.py` demo provides a simple example.
 
+## Callbacks
+
+Callback functions should execute quickly, otherwise screen refresh will be
+delayed until the callback is complete. Where a time consuming task is to be
+triggered by a callback an `asyncio` task should be launched. In the following
+sample an `LED` widget is to be cycled through various colors in response to
+a callback.
+```python
+def callback(self, button, val):
+    asyncio.create_task(self.flash_led())
+
+async def flash_led(self):
+    self.led.color(RED)
+    self.led.value(True)  # Turn on LED
+    await asyncio.sleep_ms(500)
+    self.led.color(YELLOW)
+    await asyncio.sleep_ms(500)
+    self.led.color(GREEN)
+    await asyncio.sleep_ms(500)
+    self.led.value(False)  # Turn it off. Task is complete.
+```
+The `callback()` executes fast, with the `flash_led()` running as a background
+task. For more information on asyncio, see
+[the tutorial](https://github.com/peterhinch/micropython-async/blob/master/v3/docs/TUTORIAL.md).
+
 ###### [Contents](./README.md#0-contents)
 
 ## Appendix 2 Freezing bytecode