Adding an [`issue template`](ISSUE_TEMPLATE.md) will encourage that everyone,
even a novice, reports issues in the most informative manner. This solution is
harder to miss than a section in [`CONTRIBUTING.md`](CONTRIBUTING.md) alone.
For experienced Rustaceans, it's easy to disregard/overwrite the template,
which is mostly comment, and a good reminder of some points everyone can
forget in a hurry.

content: word more precisely what information to report

contents: remove overly detailed structure from issue template.
contents: clarify what to report about specific types of issues.