Adds steps to install Rust compiler dependency

[kdmukai]

The updated cryptography==3.4.x dependency requires a Rust compiler. Updating docs with steps to install Rust compiler. Windows instructions are minimal for now.

[k9ert]

Good point. I've struggled already a bit with that in #1232
I hope that this is an intermediate problem and imho also related to the python-version and the platform people are using.
If they provide a wheel-pakage for the combination, the compiler is not needed:

• https://pypi.org/project/cryptography/3.3.2/#files
• https://pypi.org/project/cryptography/3.4.7/#files

Right? In my PR mentioned above, as far as i can remember, i moved from debian:something to ubuntu:focal and avoided the compiler doing that.
Does it make sense so mention it?

[kdmukai]

If they provide a wheel-pakage for the combination, the compiler is not needed:

@k9ert Ah, great info! I've slightly reorganized and updated the doc to reflect this.