Update issue templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+
+---
+
+**Describe the bug**
+- What did you do?
+- What happened?
+- What was expected?
+
+**Which version of tantivy are you using?**
+If "master",  ideally give the specific sha1 revision.
+
+**To Reproduce**
+
+If your bug is deterministic, can you give a minimal reproducing code?
+Some bugs are not deterministic. Can you describe with precision in which context it happened?
+If this is possible, can you share your code?

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+
+---
+
+**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.
+
+**[Optional] describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.

.github/ISSUE_TEMPLATE/question.md

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+---
+name: Question
+about: Ask any question about tantivy's usage...
+
+---
+
+Try to be specific about your use case...