Merge pull request #14675 from twbs/bug-report-guidelines

Mention HTML5 validator and Bootlint as part of troubleshooting steps

CONTRIBUTING.md

@@ -36,6 +36,10 @@ Good bug reports are extremely helpful, so thanks!
 
 Guidelines for bug reports:
 
+0. **Validate and lint your code** — [validate your HTML](http://html5.validator.nu)
+   and [lint your HTML](https://github.com/twbs/bootlint) to ensure your
+   problem isn't caused by a simple error in your own code.
+
 1. **Use the GitHub issue search** — check if the issue has already been
    reported.