Documentation for Cisco IOS device tracker (#1419)

* New cisco_ios device_tracker component docs

* Update layout

* Adding Cisco PNG

* Made smaller

source/_components/device_tracker.cisco_ios.markdown

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+---
+layout: page
+title: "Cisco IOS"
+description: "Instructions how to integrate Cisco IOS routers into Home Assistant."
+date: 2016-11-07 19:59
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: cisco.png
+ha_category: Presence Detection
+---
+
+This is a presence detection scanner for [Cisco](http://www.cisco.com) IOS devices.
+
+<p class='note warning'>
+This device tracker needs SSH to be enabled on the router.
+</p>
+
+Before using this scanner it is recommended that you lower the arp cache timeout on your router, as Cisco IOS normally comes with a 4 hour default arp cache timeout. 
+
+For example, the following commands will lower the timeout to 2 minutes on Vlan1:
+
+```bash
+# 1. use this command to see what Vlan your devices are on
+show ip arp
+
+# 2. Go into configure mode
+conf t
+
+# 3. Use the Vlan name as you see it from step 1 above
+interface Vlan1
+
+# 4. Set a new arp cache timeout
+arp timeout 120
+
+# 5. Exit
+# Press <ctrl+c> to exit configure mode
+
+# 6. Don't forget to save the new config, so that it will survive a reboot
+copy running-config startup-config
+```
+
+<p class='note warning'>
+If you have a very large number of devices on your VLan (+1000), then you may want to adjust the arp cache timeout to suit your needs. See [this discussion](https://supportforums.cisco.com/discussion/10169296/arp-timeout) to learn more. 
+</p>
+
+To use this device tracker in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+device_tracker:
+  platform: cisco_ios
+  host: ROUTER_IP_ADDRESS
+  username: YOUR_ADMIN_USERNAME
+  password: YOUR_ADMIN_PASSWORD
+```
+
+Configuration variables:
+
+- **host** (*Required*): The IP address of your router, e.g. 192.168.1.1.
+- **username** (*Required*): The username of an user with administrative privileges.
+- **password** (*Required*): The password for your given admin account.
+
+
+See the [device tracker component page](/components/device_tracker/) for instructions how to configure the people to be tracked.
+
+