inputflinger: Add IDC file documentation

This commit adds a short table for documentation with the various
properties that may be specified within an input device configuration
(IDC) file.

Bug: None
Test: Preview in Markdown previewer
Flag: DOCS_ONLY
Change-Id: Ie72b93b24187f74e79b62ca9b7f19d341875eb73

Author: Aseda Aboagye

services/inputflinger/docs/device_configuration.md

@@ -0,0 +1,10 @@
+# Input Device Configuration
+
+There are a number of properties that can be specified for an input device.
+
+|Property|Value|
+|---|----|
+|`audio.mic`|A boolean (`0` or `1`) that indicates whether the device has a microphone.|
+|`device.additionalSysfsLedsNode`|A string representing the path to search for device lights to be used in addition to searching the device node itself for lights.|
+|`device.internal`|A boolean (`0` or `1`) that indicates if this input device is part of the device as opposed to be externally attached.|
+|`device.type`|A string representing if the device is of a certain type. Valid values include `rotaryEncoder` and `externalStylus`.