Docs: Add issue reporting template (#228)

GitHub supports issue templates, see https://github.com/blog/2111-issue-and-pull-request-templates

.github/ISSUE_TEMPLATE.md

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+Please fill out the following template so we can reproduce and fix your issue as quickly as possible!
+
+Environment:
+- Preview version:
+- Browser name and version:
+- Operating system and version:
+
+Steps to reproduce the problem:
+1.
+2.
+3.
+
+What is the expected behavior? (Screenshots can be helpful here)
+
+What went wrong? (Screenshots can be helpful here)
+
+Link to application or sample code:
+
+If relevant, link to file (or attach file here):