Add an issue template to EvalML repo

As @chukarsten suggested, it could be helpful to create a template for issue-filing so that issues are clear and actionable with a repro (if applicable). Featuretools has a pretty nifty template we could use to draw inspiration from: https://github.com/alteryx/featuretools/blob/main/docs/issue_template.md

I think the template should have at least:
- Bug or feature request description
- Repro code and current behavior
- Expected behavior