Add issue template

We use single template instead of multiple templates for better usability,
as most of the time people create issues, make them to choose between templates is annoying.

Meanwhile, we minimize information load by avoiding too much info in the template.

The difference between multiple templates and single template can be seen below:

- https://help.github.com/en/articles/manually-creating-a-single-issue-template-for-your-repository

Author: liufengyun

docs/issue_template.md

@@ -0,0 +1,11 @@
+---
+name: Bug report
+about: Report a bug about the Dotty Compiler
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+- Please try to minimize the code that causes the problem
+- Feature requests should go to [lampepfl/dotty-feature-requests](https://github.com/lampepfl/dotty-feature-requests)