Clarify hardware requirements

[markafoltz]

From https://www.w3.org/2017/11/07-webscreens-minutes.html#x05:

• ACTION: @mfoltzgoogle to clarify that device requirements are driven by rendering Web content and not the network stack
• ACTION: @mfoltzgoogle to consult internally to find out if smart TVs fit within these requirements
• ACTION: @mfoltzgoogle to add hardware cryptography capabilities to device specs

[markafoltz]

We have https://github.com/w3c/openscreenprotocol/blob/main/device_specs.md which serves as our baseline.

Some of the streaming devices are quite old, but practically speaking the Raspberry PI 2 B2 is a good stand-in for any of them and I have several available for testing.

Issue #218 covers customizing the cryptographic algorithms for hardware, which I think is going to be closed out with the updated recommendations in PR #297.