Document build workflow without Docker

[Sjors]

The current README recommends the VSCode extension, which in turn uses Docker. I can't get that to work so far, see #347.

It would be great to have instructions (just Ubuntu is fine) for building without Docker.

I was able to get Speculos up and running, which ships with an (old?) version of the app. So I guess once I can build an arbitrary version, it's just a matter of copying it over?

[bigspider]

Yes, once you get a binary and have speculos installed, running speculos bin/app.elf (or wherever you have the binary) should work.