Create ISSUE_TEMPLATE.md

.github/ISSUE_TEMPLATE.md

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+Too many issues will kill our team's development velocity, drastically.
+Make sure you have checked all steps below.
+
+### Prerequisite
+* [ ] Make sure your problem is not listed in [the common build problems](https://github.com/pyenv/pyenv/wiki/Common-build-problems).
+* [ ] Make sure no duplicated issue has already been reported in [the pyenv issues](https://github.com/pyenv/pyenv-virtualenv/issues). You should look in closed issues, too.
+* [ ] Make sure you are not asking us to help solving your specific issue.
+  * GitHub issues is opened mainly for development purposes. If you want to ask someone to help solving your problem, go to some community site like [Gitter](https://gitter.im/yyuu/pyenv), [StackOverflow](https://stackoverflow.com/questions/tagged/pyenv), etc.
+* [ ] Make sure your problem is not derived from packaging (e.g. [Homebrew](https://brew.sh)).
+  * Please refer to the package documentation for the installation issues, etc.
+* [ ] Make sure your problem is not derived from other plugins.
+  * This repository is maintaining the `pyenv-virtualenv` plugin only. Please refrain from reporting issues of other plugins here.
+
+### Description
+- [ ] Platform information (e.g. Ubuntu Linux 20.04):
+- [ ] OS architecture (e.g. amd64):
+- [ ] pyenv version:
+- [ ] pyenv-virtualenv version:
+- [ ] Python version:
+- [ ] virtualenv version (if installed):
+- [ ] Please attach debug log of a faulty Pyenv invocation as a gist
+  * You can turn on verbose debug logging using by setting `PYENV_DEBUG=1`, e.g. `env PYENV_DEBUG=1 pyenv install -v 3.6.4`