Add GitHub issue template

Refs twbs#19260
[ci skip]

ISSUE_TEMPLATE.md

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+If you're reporting a bug:
+* Have you [searched for duplicate issues, including closed issues?]( https://github.com/twbs/bootstrap/issues?utf8=%E2%9C%93&q=is%3Aissue ): 
+* Have you [validated]( http://validator.w3.org/nu/ ) your page's HTML?: 
+* Have you [linted]( https://github.com/twbs/bootlint#in-the-browser ) your page's HTML?: 
+* What version of Bootstrap are you using?: 
+  * If you're not using the latest version (3.3.6 or 4.0.0-alpha.2), please try upgrading first.
+* Which OS are you using?: 
+  * [Windows, Mac OS X, Android, iOS, Win10 Mobile]
+* Which browser are you using?: 
+  * [Chrome, Firefox, Safari, IE, MS Edge, Opera 15+, Android Browser]
+* What versions of your OS and browser are you using?: 
+* What's the URL of your [reduced test case]( https://css-tricks.com/reduced-test-cases/ )?: 
+  * If you don't provide a reduced test case, your issue may get summarily closed.
+  * Please use [BootstrapCDN]( https://www.bootstrapcdn.com ) and [JS Bin]( https://jsbin.com ) (or JSFiddle, CodePen, etc.) for your testcase.
+  * Full, non-simplified production webpages don't qualify as reduced test cases.
+
+---
+
+If you need help debugging your specific webpage or have a "How do I...?" question,
+DON'T open an issue. Ask for help on StackOverflow, IRC, or Slack instead.
+See https://github.com/twbs/bootstrap/blob/master/README.md#community for links to those personal support forums.
+
+---
+
+If you have a feature request:
+* Have you [searched for duplicate issues, including closed issues?]( https://github.com/twbs/bootstrap/issues?utf8=%E2%9C%93&q=is%3Aissue ): 
+* Note that your request will only be considered for Bootstrap 4.
+  * [Bootstrap 3 is no longer receiving new features.]( https://github.com/twbs/bootstrap/blob/master/CONTRIBUTING.md#feature-requests )
+* Please describe your idea in as much detail as possible. (Terse or unclear requests may get summarily closed.)