add a issue template

Author: yihui

.github/ISSUE_TEMPLATE.md

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+
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+<!--
+Please keep the portion below in your issue. Your issue will be closed if any of the boxes is not checked. In certain (rare) cases, you may be exempted if you give a brief explanation (e.g., you are only making a suggestion for improvement). Thanks!
+-->
+
+---
+
+By filing an issue to this repo, I promise that
+
+- [ ] I have fully read the issue guide at https://yihui.name/issue/.
+- [ ] I have provided the necessary information about my issue.
+    - If I'm asking a question, I have already asked it on Stack Overflow or RStudio Community, waited for at least 24 hours, and included a link to my question there.
+    - If I'm filing a bug report, I have included a minimal, self-contained, and reproducible example, and have also included `xfun::session_info('DT')`. I have upgraded all my packages to their latest versions (e.g., R, RStudio, and R packages), and also tried the development version: `remotes::install_github('rstudio/DT')`.
+    - If I have posted the same issue elsewhere, I have also mentioned it in this issue.
+- [ ] I have learned the Github Markdown syntax, and formatted my issue correctly.