Port twbs#19706 to v3

[ci skip]

ISSUE_TEMPLATE.md

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+Before opening an issue:
+
+- [Search for duplicate or closed issues](https://github.com/twbs/bootstrap/issues?utf8=%E2%9C%93&q=is%3Aissue)
+- [Validate](http://validator.w3.org/nu/) and [lint](https://github.com/twbs/bootlint#in-the-browser) any HTML to avoid common problems
+- Prepare a [reduced test case](https://css-tricks.com/reduced-test-cases/) for any bugs
+- Read the [contributing guidelines](https://github.com/twbs/bootstrap/blob/master/CONTRIBUTING.md)
+
+When asking general "how to" questons:
+
+- Please do not open an issue here
+- Instead, ask for help on [StackOverflow, IRC, or Slack](https://github.com/twbs/bootstrap/blob/master/README.md#community)
+
+When reporting a bug, include:
+
+- Operating system and version (Windows, Mac OS X, Android, iOS, Win10 Mobile)
+- Browser and version (Chrome, Firefox, Safari, IE, MS Edge, Opera 15+, Android Browser)
+- Reduced test cases and potential fixes using [JS Bin](https://jsbin.com)
+
+When suggesting a feature, include:
+
+- As much detail as possible for what we should add and why it's important to Bootstrap
+- Relevant links to prior art, screenshots, or live demos whenever possible