[meta] Add guidance for how to write good new issues

[justinfagnani]

For people who aren't regular contributors here it would be really useful to have some guidelines on how to open a good issue.

There is a lot of variation in issue titles, though an obvious common structure for most of them. Non-working group members might not know what categories to use, or whether a member will clean up the title later if it gets discussed.

Should new issues include a prefix like [proposal] or [idea]? Should "proposal" be used informally, or is it a more formal term here that includes specific choices fora change?

Likewise, with so many issues it's common for other repositories for new issues to include the search terms they used to try to find an existing issue first.

There could be an issue template, or just guidance on https://github.com/w3c/csswg-drafts/blob/main/CONTRIBUTING.md

[kbabbitt]

My observations:

• The only consistent convention I've seen with issue naming is to prefix with the shortname(s) of the relevant spec(s) in brackets. One might argue that GitHub tagging is a cleaner way to relate issues to specs, though I do find it convenient to have shortnames in issue titles when I'm scanning email notifications.
• I've seen issues that introduce new ideas or features both labeled "Proposal" and not. It doesn't appear to have a specific meaning other than when it's used to introduce a "proposed resolution" meaning "thing the author would like the working group to resolve on."

I think a "good issue" contains the following elements:

• Link to and/or quote of relevant existing spec text, where applicable
• Description of what the author would like added, clarified, or changed, and why
• Suggested changes if the issue author has something in mind already
• Markup snippets / test cases / example renderings are usually quite helpful