Document gemspec metadata

[lostapathy]

rubygems/rubygems.org#724 then rubygems/rubygems.org#718 and rubygems/rubygems.org#1234 discuss using gemspec metadata to populate urls in the gems website. This patch updates the docs to reflect the conventions that were codified in https://github.com/rubygems/rubygems.org/blob/master/app/models/links.rb#L3

I don't think this fully resolves the open issues, but it should at least bring the guides up to the current status.

[bundlerbot]

☔ The latest upstream changes (presumably a9e2c02) made this pull request unmergeable. Please resolve the merge conflicts.

[segiddins]

This seems to be handled by a9e2c02#diff-a0ed24268b853e4412f766326125de00R316

[lostapathy]

Yes, that commit covers it, thanks @segiddins. I didn't realize, but it appears that is generated from the rubygems source, correct?

I feel like this repo might deserve it's own contributor guide to provide some meta-documentation on how this all comes together on guides.rubygems.com. If there's interest I can start on that, although not sure what to call it since contributing.md is already used to generate website content

[segiddins]

Maybe .github/CONTRIBUTING.md?

[lostapathy]

Thinking about this some more, I think our best course of action is to rename the current contributing.md to contributing-to-rubygems.md, add a new CONTRIBUTING.md, and tell jekyll to ignore the new file.

Jekyll will still build the gems site as it does now, but putting the CONTRIBUTING.md file in the canonical location should make it more likely that people actually discover it.

I'll try to put that PR together separate. @segiddins I may be in touch to make sure I understand how everything is generated so I can keep the PR reasonably clean on the first try.