Add CONTRIBUTING.md and a code of conduct

BaryoVM has a README, a USAGE guide and a VISION doc, which is more than most tools this age. What it lacks is anything telling someone how to contribute: no CONTRIBUTING, no code of conduct, no issue templates. **Scope** - `CONTRIBUTING.md` — how to build, how to run the tests, branch naming, what a good PR looks like - `CODE_OF_CONDUCT.md` - Issue templates for a bug and a feature Mirror [barakoCMS's](https://github.com/BaryoDev/barakoCMS/blob/master/CONTRIBUTING.md) so the two projects feel like one house rather than two unrelated repos. **Worth stating explicitly in CONTRIBUTING:** this tool runs commands against real machines over SSH, so a change to command construction can break someone's deployment. That is why the golden-file tests in #2 matter.