Add CONTRIBUTING.md file

[sjspielman]

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

[jashapiro]

• 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

We might as well add devtools to renv for convenience. There isn't really any reason not to, as long as we add any dependencies.R to .Rbuildignore