Bring Py-FSRS up to recommended community standards (#93)

* add code of conduct

* add security policy

* set up github issue templates

* add pull request template

.github/ISSUE_TEMPLATE/bug-report.md

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+---
+name: Bug Report
+about: Report a bug and provide reproduction steps
+title: ""
+labels: ""
+assignees: ""
+---
+
+**Which version of py-fsrs?**
+
+<!-- specify which version of py-fsrs you're using -->
+
+**Bug description:**
+
+<!-- describe the bug here -->
+
+**Steps to reproduce the bug:**
+
+<!-- provide a minimum code example to reproduce the bug -->
+
+```python
+
+```

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true
+contact_links:
+    - name: Questions
+      url: https://github.com/open-spaced-repetition/py-fsrs/discussions
+      about: Please ask general questions here instead of opening an issue.

.github/PULL_REQUEST_TEMPLATE.md

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+<!--If this PR resolves an issue, link its number below. Otherwise, delete the line.-->
+Closes #ISSUE_NUMBER

CODE_OF_CONDUCT.md

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+# Code of Conduct
+
+Always be respectful, assume good intent and try to stay on topic as much as possible.

SECURITY.md

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+# Security policy
+
+## Supported versions
+
+Only the latest version of py-fsrs will be supported with security updates. You can find the lastest version of py-fsrs [here](https://github.com/open-spaced-repetition/py-fsrs/releases/latest).
+
+## Reporting a security vulnerability
+
+If you find a security vulnerability with py-fsrs, we'd like to know!
+
+For non-severe vulnerabilites, please just open a normal [issue](https://github.com/open-spaced-repetition/py-fsrs/issues).
+
+For more severe security vulnerabilities, please report it [here](https://github.com/open-spaced-repetition/py-fsrs/security/advisories/new).