Flesh out `principles.md`

#94 adds `principles.md` to make explicit important internal coding conventions used in rextendr. That PR limited the contents of `principles.md` to user communication and errors. However, there are probably other conventions (or emerging conventions waiting to be codified) that would do well to be included.

Two particularly good examples of documents like this are in [usethis](https://github.com/r-lib/usethis/blob/master/principles.md) and [targets](https://books.ropensci.org/targets-design/index.html). These are both very well designed packages, and I think the principled approach to design helped get them there.

- [x] Communicating with users
- [x] Throwing errors
- [ ] `to_toml()`
- [ ] Working with paths, `pretty_rel_path()`
- [ ] Testing, including helper functions + snapshot testing

Read principles.md and the prior #94 change, then review the unchecked topics: to_toml(), pretty_rel_path(), and testing helpers or snapshot testing. Compare the linked usethis and targets examples; done means principles.md documents the agreed conventions for these areas.

Written by the indexing model from the issue text.