design.md Update

[tagatac]

Is your feature request related to a problem? Please describe.
It's not clear what the design of wslink is currently. design.md was written in 2017, and seems to contain several notes about possible implementations. Thus, I cannot tell without reading the code what design was actually implemented and how it has changed since 2017.

Describe the solution you'd like
An update to design.md to make it clear what has been implemented through version 1.11, and a removal of any notes that are no longer relevant.

Describe alternatives you've considered
Remove the design doc, and tell users to read the code instead (not ideal).

Additional context
Thanks for the project!

[jourdain]

The description and background still stand:

• Python server / JavaScript client
• RPC using Python decorator (JS can call Python. Not symmetric) with support for args and kwargs.
• Pub/Sub can be done from either side.
• Binary attachments can be added to RPC and Pub/Sub messages

The rest are just implementation details.

The big difference when we moved to v1.0.0+ was to use Python 3.6+ only and asyncio as core. The first initial backend implementation is based on aiohttp. A generic backend is available and tested by enabling tornado via the generic one.