chore: Add CONTRIBUTING

.github/CONTRIBUTING.md

+# Contributing
+
+## Making Changes
+
+This project follows [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/) to allow for [Semantic Versioning](https://semver.org/).
+
+- [Conventional commit types explained](https://daily-dev-tips.com/posts/git-basics-conventional-commits/)
+
+### Adding new functionality
+
+New functions should have test cases.
+
+``` r
+usethis::use_r("NEW_FUNCTION_NAME_HERE")
+usethis::use_test("NEW_FUNCTION_NAME_HERE")
+```
+
+### Changing `DESCRIPTION`
+
+Cleanup after making changes to `DESCRIPTION`.
+
+``` r
+usethis::use_tidy_description()
+```
+
+## Preparing your Pull Request
+
+Make sure documentation is up-to-date and checks pass on your machine.
+
+``` r
+styler::style_pkg()
+devtools::document()
+devtools::check()
+```