.github: add issue templates (#124)

* add bug-report template

* add feature-request template

* add question template

.github/ISSUE_TEMPLATE/bug-report.md

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+---
+name: "\U0001F41B Bug Report"
+labels: "bug"
+about: Something isn't working as expected
+
+---
+
+## Bug Report
+
+Please answer these questions before submitting your issue. Thanks!
+
+1. What did you do? If possible, provide a recipe for reproducing the error.
+
+2. What did you expect to see?
+
+3. What did you see instead?
+
+4. Versions of the cluster
+
+    - TiDB-Lightning version (run `tidb-lightning -V`):
+
+        ```
+        (paste TiDB-Lightning version here)
+        ```
+        
+    - TiKV-Importer version (run `tikv-importer -V`)
+
+        ```
+        (paste TiKV-Importer version here)
+        ```
+        
+    - TiKV version (run `tikv-server -V`):
+
+        ```
+        (paste TiKV version here)
+        ```
+
+    - TiDB cluster version (execute `SELECT tidb_version();` in a MySQL client):
+
+        ```
+        (paste TiDB cluster version here)
+        ```
+
+    - Other interesting information (system version, hardware config, etc):
+
+        >
+        >
+
+5. Operation logs
+   - Please upload `tidb-lightning.log` for TiDB-Lightning if possible
+   - Please upload `tikv-importer.log` from TiKV-Importer if possible
+   - Other interesting logs
+
+6. Configuration of the cluster and the task
+   - `tidb-lightning.toml` for TiDB-Lightning if possible
+   - `tikv-importer.toml` for TiKV-Importer if possible
+   - `inventory.ini` if deployed by Ansible
+
+7. Screenshot/exported-PDF of Grafana dashboard or metrics' graph in Prometheus for TiDB-Lightning if possible

.github/ISSUE_TEMPLATE/feature-request.md

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+---
+name: "\U0001F680 Feature Request"
+labels: "feature-request"
+about: I have a suggestion
+
+---
+
+## Feature Request
+
+**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 feature 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. -->
+
+**Teachability, Documentation, Adoption, Optimization:**
+<!-- If you can, explain some scenarios how users might use this, situations it would be helpful in. Any API designs, mockups, or diagrams are also helpful. -->

.github/ISSUE_TEMPLATE/question.md

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+---
+name: "\U0001F914 Question"
+labels: "question"
+about: Usage question that isn't answered in docs or discussion
+
+---
+
+## Question
+
+Before asking a question, make sure you have:
+
+- Searched existing Stack Overflow questions.
+- Googled your question.
+- Searched open and closed [GitHub issues](https://github.com/pingcap/tidb-lightning/issues?q=is%3Aissue)
+- Read the documentation:
+    * [Detailed documentation](https://pingcap.com/docs/tools/lightning/overview-architecture/)
+    * [简体中文文档](https://pingcap.com/docs-cn/tools/lightning/overview-architecture/)