README: more references, DT history, etc

[orangecms]

more references to add:

• https://elinux.org/Device_Tree_Mysteries#kernel_usage
• ...?

I gave a few talks in which I was briefly explaining device trees, e.g., Hack the Gadget! (slides). This specific one is what dtvis originates from.

The DT history itself goes back to IEEE 1275 (OpenFirmware).

[m0veax]

Referenced that sources in my PR