Update instructions and code of conduct

[wlandau]

This PR adds a code of conduct from usethis::use_code_of_conduct(). I also added the following line under "Examples of unacceptable behavior include:"

• Maliciously attributing the wrong owner or repository in the URL of a package entry contributed to https://github.com/r-releases/r-releases/tree/main/packages.

In the README, I linked to the code of conduct, explained JSON format, and generally cleaned up the wording.

@shikokuchuo, would you please review?

[wlandau]

Thanks!

[shikokuchuo]

Perfect!