Add issue templates

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**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 or screenshots about the feature request here.

.github/ISSUE_TEMPLATE/xsd-issue.md

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+---
+name: XSD issue
+about: Describe issues related to the .xsd schema files.
+title: ''
+labels: xsd
+assignees: Kiyotoko
+
+---
+
+**Related file**
+Briefly state which file is involved.
+
+**The problem**
+Explain clearly what the problem is. Common problems can be that a tag, documentation or the data type is incorrect.
+
+**Your solution**
+If you already have a solution or a suggestion for improvement, please post it here.