Add auto-water setting and disable by default

Adds a new setting - auto_water - and disables by default to reflect the basic Grow kit being monitor-only.

Adds documentation for the settings file.

examples/README.md

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+# Watering Settings
+
+The `grow-monitor-and-water.py` example can monitor and, optionally, automatically water all three Grow channels.
+
+It's configured using a settings file - `water.yml` - that looks like the following:
+
+```yaml
+channel1:
+        water_level: 0.8
+        warn_level: 0.2
+        pump_speed: 0.7
+        pump_time: 0.7
+        auto_water: False
+        icon: icons/flat-4.png
+channel2:
+        water_level: 0.8
+        warn_level: 0.2
+        pump_speed: 0.7
+        pump_time: 0.7
+        auto_water: False
+channel3:
+        water_level: 0.8
+        warn_level: 0.2
+        pump_speed: 0.7
+        pump_time: 0.7
+        auto_water: False
+general:
+        alarm_enable: True
+        alarm_interval: 1.0
+```
+
+By default auto-watering is disabled and an alarm will sound every 1s if the `warn_level` is reached.
+
+## Channel Settings
+
+* `water_level` - The level at which auto-watering should be triggered (soil saturation from 0.0 to 1.0)
+* `warn_level` - The level at which the alarm should be triggered (soil saturation from 0.0 to 1.0)
+* `pump_speed` - The speed at which the pump should be run (from 0.0 low speed to 1.0 full speed)
+* `pump_time` - The time that the pump should run for (in seconds)
+* `auto_water` - Whether to run the attached pump (True to auto-water, False for manual watering)
+* `icon` - Optional icon image for the channel, see the icons directory for images.
+
+## General Settings
+
+* `alarm_enable` - Whether to enable the alarm
+* `alarm_interval` - The interval at which the alarm should beep (in seconds)