Add proximity docs

source/_components/proximity.markdown

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+---
+layout: component
+title: "Proximity"
+description: "Instructions how to setup Proximity monitoring within Home Assistant."
+date: 2016-02-07 10:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: home-assistant.png
+ha_category: Organization
+---
+
+The `proximity` component allows you to monitor the proximity of devices to a particular [zone](/components/zone/) and the direction of travel. The result is an entity created in Home Assistant which maintains the proximity data.
+
+This component is useful to reduce the number of automation rules required when wanting to perform automations based on locations outside a particular zone. The [zone](/components/automation/#zone-trigger) and [state](/components/automation/#state-trigger) based triggers allow similar control but the number of rules grows exponentially when factors such as direction of travel need to be taken into account.
+
+Some examples of its use include:
+
+- Increase thermostat temperature as you near home
+- Decrease temperature the further away from home you travel
+
+The Proximity entity which is created has the following values:
+
+- `state`: Distance from the monitored zone (in km)
+- `dir_of_travel`: Direction of the closest device to the monitoed zone. Values are:
+  - 'not set'
+  - 'arrived'
+  - 'towards'
+  - 'away_from'
+  - 'unknown'
+  - 'stationary'
+- `dist_to_zone`: Distance from the monitored zone (in km)
+
+To enable this component in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+proximity:
+  zone: home
+  ignored_zones:
+    - twork
+    - elschool
+  devices:
+    - device_tracker.nwaring_nickmobile
+    - device_tracker.eleanorsiphone
+    - device_tracker.tsiphone
+  tolerance: 50
+```
+
+Configuration variables:
+
+- **zone** (*Optional*): The zone to which this component is measuring the distance to. Default is the home zone.
+- **ignored_zones** array (*Optional*): Where proximity is not calculated for a device (either the device being monitored or ones being compared (e.g. work or school).
+- **devices** array (*Optional*):  A list of devices to compare location against to check closeness to the configured zone.
+- **tollerance** (*Optional*): The tolerance used to calculate the direction of travel in metres to filter out small GPS coordinate changes.
+