EVENTS.md: Add TOC.

Author: peterhinch

v3/docs/EVENTS.md

@@ -7,6 +7,8 @@ application logic.
 This document assumes familiarity with `uasyncio`. See [official docs](http://docs.micropython.org/en/latest/library/uasyncio.html) and
 [unofficial tutorial](https://github.com/peterhinch/micropython-async/blob/master/v3/docs/TUTORIAL.md).
 
+# 0. Contents
+
  1. [An alternative to callbacks in uasyncio code](./EVENTS.md#1-an-alternative-to-callbacks-in-uasyncio-code)  
  2. [Rationale](./EVENTS.md#2-rationale)  
  3. [Device driver design](./EVENTS.md#3-device-driver-design)  
@@ -23,7 +25,7 @@ This document assumes familiarity with `uasyncio`. See [official docs](http://do
   6.2 [EButton](./EVENTS.md#62-ebutton)  
        6.2.1 [The suppress constructor argument](./EVENTS.md#621-the-suppress-constructor-argument)  
        6.2.2 [The sense constructor argument](./EVENTS.md#622-the-sense-constructor-argument)  
-[Appendix 1 Polling](./EVENTS.md-appendix-1-polling)  
+[Appendix 1 Polling](./EVENTS.md#-appendix-1-polling)  
 
 # 1. An alternative to callbacks in uasyncio code
 
@@ -69,6 +71,8 @@ Note the `Stream` mechanism provides another approach which works well with
 devices such as sockets and UARTs. It is less well suited to handling arbitrary
 events, partly because it relies on polling under the hood.
 
+###### [Contents](./EVENTS.md#0-contents)
+
 # 2. Rationale
 
 Consider a device driver `Sensor` which has a bound `Event` object `.ready`.
@@ -119,6 +123,8 @@ callbacks:
  execution reaches `.wait()` before the `Event` is cleared, it will not pause.
  If the `Event` is cleared, it will pause until it is set again.
 
+###### [Contents](./EVENTS.md#0-contents)
+
 # 3. Device driver design
 
 This document introduces the idea of an event-like object (ELO). This is an
@@ -146,6 +152,8 @@ Drivers exposing `Event` instances include:
  * [Switch][5m] Similar but interfaces also expose callbacks.
  * [Pushbutton][6m]
 
+###### [Contents](./EVENTS.md#0-contents)
+
 # 4. Primitives
 
 Applying `Events` to typical logic problems requires two new primitives:
@@ -188,6 +196,8 @@ await WaitAll((event1, event2, WaitAny(event3, event4))).wait()
 This will pause until `event1` and `event2` and either `event3`or `event4` have
 been set.
 
+###### [Contents](./EVENTS.md#0-contents)
+
 # 5. Event based programming
 
 ## 5.1 Use of Delay_ms
@@ -281,6 +291,8 @@ async def main():
             # Both timers running: run "short press" code
 ```
 
+###### [Contents](./EVENTS.md#0-contents)
+
 ## 5.3 Application example
 
 A measuring instrument is started by pressing a button. The measurement
@@ -314,6 +326,8 @@ async def foo():
             # Normal outcome, process readings
 ```
 
+###### [Contents](./EVENTS.md#0-contents)
+
 # 6. Drivers
 
 This document describes drivers for mechanical switches and pushbuttons. These
@@ -355,6 +369,8 @@ Bound objects:
 
 Application code is responsible for clearing the `Event` instances.
 
+###### [Contents](./EVENTS.md#0-contents)
+
 ## 6.2 EButton
 
 This extends the functionality of `ESwitch` to provide additional events for
@@ -443,6 +459,8 @@ the voltage on the pin is gnd.
 Whenever the pin value changes, the new value is compared with `sense` to
 determine whether the button is closed or open.
 
+###### [Contents](./EVENTS.md#0-contents)
+
 # Appendix 1 Polling
 
 The primitives or drivers referenced here do not use polling with the following