Refresh instructions for use on *nix and set up build on e.g. travis

[jmp75]

Looking at a notification from a pull request that's actually a usage question. Instructions do need a refresh, and wirth trying to set up travis.yml also for personal exercise.

[jmp75]

https://docs.travis-ci.com/user/languages/csharp/#testing-against-mono-and-net-core

http://blog.buildamation.com/2018/01/14/travisci-and-appveyor-for-buildamation/