Add PR template (#16)

.github/pull_request_template.md

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+# Objective
+
+- Describe the aim of this pull request.
+- If this closes an issue, type "Closes #X".
+
+## Solution
+
+- Describe the steps taken to achieve the objective.
+
+---
+
+## Changelog
+
+> This section is optional.
+> If this PR only fixes a trivial bug
+> or has no externally-visible impact in the API,
+> then feel free to delete this section. 
+
+- This repository follows the [_Keep A Changeog_][KAC] format.
+- Additionally, this follows [_Semantic Versioning_][SemVer].
+
+[KAC]: https://keepachangelog.com/en/1.0.0/
+[SemVer]: https://semver.org/
+
+## Migration Guide
+
+> This section is optional.
+> If this PR creates a breaking change,
+> please indicate the steps needed
+> for the user to migrate to the new version.
+> Otherwise, feel free to delete this section.
+
+- A guideline to determine whether the changes are breaking can be found [here](https://doc.rust-lang.org/cargo/reference/semver.html).