docs: "Portable; if vcsh is not installed on a machine, just copy the one POSIX script over"

[RichiH]

No description provided.

[alerque]

Closing this as a duplicate of #202. I definitely don't want to document this as a feature right now because it might not be true much longer. If I get my way with a configure/build system this might not be quite true any more (I'll list that as a downside in notes on the PR for review). It would work between very similar systems (i.e. scping the script from one GNU system to another or or one BSD to another would probably work, but scping my script from an Arch Linux box to a Windows or macOS machine might fail because the paths to dependencies that were discovered at compile time instead of run time could be different).

On the other hand a 'portable' or bootstrappable variant might still be something that could be explicitly built or otherwise arranged for, so tracking this on the other issue makes sense to me.