Add a GitHub issue template with suggestions to improve issue quality…

… (based on a proposal by @nschonni).

.github/ISSUE_TEMPLATE/markdownlint-issue-template.md

@@ -0,0 +1,40 @@
+---
+name: markdownlint issue template
+about: This template helps report issues with the markdownlint family of tools.
+title: ''
+labels: ''
+assignees: ''
+---
+
+<!--
+
+Thank you for taking the time to report an issue!
+
+When deciding where to open an issue, please note there are multiple projects under the markdownlint umbrella:
+
+- https://github.com/DavidAnson/markdownlint : This is the core JavaScript/Node.js library and is used by other tools. Most issues with implementation and rule behavior belong here.
+- https://github.com/igorshubovych/markdownlint-cli : This is the original CLI for markdownlint. Issues specific to CLI belong here.
+- https://github.com/DavidAnson/markdownlint-cli2 : This is a newer CLI for markdownlint and is used by other tools. Issues specific to CLI2 belong here.
+- https://github.com/DavidAnson/vscode-markdownlint : This is the Visual Studio Code extension for markdownlint. Issues specific to VS Code belong here.
+- https://github.com/DavidAnson/markdownlint-cli2-action : This is a GitHub Action for markdownlint. Issues specific to the Action belong here.
+- https://github.com/markdownlint/markdownlint : This is the original markdownlint implementation for Ruby. All Ruby-related issues belong here.
+
+Before creating an issue, it's a good practice to search existing issues for something similar. If your issue has already been reported, please update the existing one with any new information. It's also good to review the documentation for any relevant details.
+
+When describing an issue, the following information is helpful:
+
+- What did you do?
+- What did you expect to happen?
+- What actually happened?
+- What messages or errors were there?
+- How can the issue be reproduced?
+- What version were you using?
+- What operating system were you using?
+
+The simplest demonstration of a problem is the most helpful. Small examples can be pasted into the issue description. (Be sure to paste as code so GitHub doesn't render the example in Markdown.) For larger examples, linking to a repository or file is more appropriate.
+
+Before proposing a new rule, please review the existing suggestions: https://github.com/DavidAnson/markdownlint/issues?q=is%3Aissue+is%3Aopen+label%3A%22new+rule%22
+
+Thank you!
+
+-->