Add start of device tag documentation

We can tag devices in various ways; for different purpose. Start
documenting what those are (even if they're not rolled out yet)

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>

content/device_tags.md

+---
+title: Device tags
+---
+
+Lava supports [device tags](https://lava.collabora.co.uk/static/docs/v2/glossary.html#term-device-tag)
+as a way to add criteria/requirements to devices on which a job can run. This
+document explains the various tags in use and planned in the Collbora Lava
+instance.
+
+## Support level tags ##
+
+**Not deployed yet**
+
+This group of tags represents the [support levels](support) of a device Each
+tag represent that the device supports at _least_ that level of support. While
+the requirements for a given support mostly refer to the device-type as a whole
+a specific device may have a *lower* support level itself due to its setup
+(e.g. different hardware setup or experimental
+lab).
+
+* `support-experimental`: This device (type) has "experimental" support level or higher
+* `support-supported`: This device (type) has "supported" support level or higher
+* `support-priority`: This device (type) has "priority" support level
+
+## Infrastructure tags ##
+
+* `nfsroot`: **Not deployed yet** Dispatcher and network is setup for NFS root usage
+
+## Peripheral tags ##
+
+This group of tags represent periphals that are connected or available on a
+device.
+
+* `sata`: Device has a sata disk attached