Add a GitHub issue template.

.github/ISSUE_TEMPLATE.md

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+Hello, and thanks for opening a new issue about Rocket!
+
+Before opening your issue, we ask that you search through existing issues and
+pull requests to see if your bug report, concern, request, or comment has
+already been addressed. Ensure to search through both open and closed issues and
+pull requests. If this is a question, feature request, or general comment,
+please ensure that you read the relevant sections of the documentation before
+posting your issue. Finally, consider asking questions on IRC or Matrix before
+opening an issue.
+
+If you feel confident that your issue is unique, please including the following
+information, selecting the category that best describes your issue:
+
+## Bug Reports
+
+Bug reports _must_ include:
+
+  1. The version of Rocket you're using. Ensure it's the latest, if possible.
+
+  2. A brief description of the bug that includes:
+    * The nature of the bug.
+    * When the bug occurs.
+    * What you expected vs. what actually happened.
+
+  3. How you discovered the bug. Short test cases are especially useful.
+
+  4. Ideas, if any, about what Rocket is doing incorrectly.
+
+## Questions
+
+Any questions _must_ include:
+
+  1. The version of Rocket this question is based on, if any.
+
+  2. What steps you've taken to answer the question yourself.
+
+  3. What documentation you believe should include an answer to this question.
+
+## Feature Requests
+
+Feature requests _must_ include:
+
+  1. Why you believe this feature is necessary.
+
+  2. A convincing use-case for this feature.
+
+  3. Why this feature can't or shouldn't exist outside of Rocket.
+
+## General Comments
+
+Feel free to comment at will. We simply ask that your comments are well
+constructed and actionable. Consider whether IRC or Matrix would be a better
+venue for discussion.