Added an issue template for Github (#6205)

The MD file has been added in the root folder, as CONTRIBUTING.md

issue_template.md

+If you want to report a build error for some package, or a bug in core please use the following template as a guideline:
+
+-----
+
+Summary
+
+### Expected Result
+
+What you expected
+
+### Actual Result
+
+What happened instead
+
+### Steps to reproduce the issue
+
+```console
+$ spack <command1> <spec>
+$ spack <command2> <spec>
+...
+```
+
+### Information on your system
+
+This includes:
+
+ 1. which platform you are using
+ 2. any relevant configuration detail (custom `packages.yaml` or `modules.yaml`, etc.)
+
+-----
+
+We encourage you to try, as much as possible, to reduce your problem to the minimal example that still reproduces the issue. That would help us a lot in fixing it quickly and effectively!
+
+If you want to ask a question about the tool (how to use it, what it can currently do, etc.), try the `#general` channel on our Slack first. We have a welcoming community and chances are you'll get your reply faster and without opening an issue.
+
+Other than that, thanks for taking the time to contribute to Spack!
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