docs: Add link to blinky

The blinky example application is a good demonstrator for how to write a
new application that uses the Open IoT SDK. The examples included with
this repo, at present, only demonstrate use of the libraries shipped via
the SDK, rather than e.g. how to structure your CMakeLists.txt.

Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>