establish repo-wide config files

- [x] `DESCRIPTION`
- [x] `README.md` with badges and installation instructions
- [x] `.gitignore`
- [x] `.gitattributes`
- [x] `.Rbuildignore`
- [ ] code of conduct https://help.github.com/articles/adding-a-code-of-conduct-to-your-project/
- [x] license (eg, GPL)