Add CONTRIBUTING.md file

To support future development of this package, both by Data Lab staff and external contributors who want to join in, we would benefit from a `CONTRIBUTING.md` file. A limited list of what this file should contain, based on some discussion with @cansavvy :

- Explain how this repo/package relates to `OpenScPCA-analysis`, but also...
- Clarify that this is indeed a standalone package that doesn't require `OpenScPCA-analysis` for development, and make sure folks are aware of the test data in the test suite that can be used for development
- Explain how `renv` is used (and not used) here, including that `devtools` isn't tracked and may need to be installed fresh - but don't worry, this won't hurt `renv` since it won't get snapshotted in

Create CONTRIBUTING.md and use the issue's checklist as the starting point. Review the package's relationship to OpenScPCA-analysis, the test-suite data available for development, and the stated renv and devtools workflow; the work is done when each point is clearly documented for staff and external contributors.

Written by the indexing model from the issue text.