Update issue templates inspired/based on dask

.github/ISSUE_TEMPLATE/bug-report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!-- Please include a self-contained copy-pastable example that generates the issue if possible.
+
+Please be concise with code posted. See guidelines below on how to provide a good bug report:
+
+- Craft Minimal Bug Reports: http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports
+- Minimal Complete Verifiable Examples: https://stackoverflow.com/help/mcve
+
+Bug reports that follow these guidelines are easier to diagnose, and so are often handled much more quickly.
+-->
+
+**What happened**:
+
+**What you expected to happen**:
+
+**Minimal Complete Verifiable Example**:
+
+```python
+# Put your MCVE code here
+```
+
+**Anything else we need to know?**:
+
+**Environment**:
+
+<details><summary>Output of <tt>xr.show_versions()</tt></summary>
+
+<!-- Paste the output here xr.show_versions() here -->
+
+
+</details>

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true
+contact_links:
+  - name: General Question
+    url: https://stackoverflow.com/questions/tagged/python-xarray
+    about: "If you have a question like *How do I append to an xarray.Dataset?* then please ask on Stack Overflow using the #python-xarray tag."

.github/ISSUE_TEMPLATE/feature-request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!-- Please do a quick search of existing issues to make sure that this has not been asked before. -->
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context about the feature request here.