.github/CODE_OF_CONDUCT.md is a possibly invalid file URL

[jmgirard]

The use_tidy_coc() function adds a link to .github/CODE_OF_CONDUCT.md to README.md. However, this now triggers a NOTE on CRAN check and prevents CRAN acceptance:

* checking CRAN incoming feasibility ... NOTE
Found the following (possibly) invalid file URI:
  URI: .github/CODE_OF_CONDUCT.md
    From: README.md

From CRAN manual inspection:

Please replace by a fully specified URL. Please fix and resubmit.

So it looks like this kind of relative URL may no longer be accepted by CRAN check.

[jennybc]

Duplicate of #772