add contributing guidelines

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Please make sure to read the Pull Request Guidelines:
+https://github.com/swing-opensource/swingmusic/.github/CONTRIBUTING.md#pull-request-guidelines
+-->
+
+<!-- PULL REQUEST TEMPLATE -->
+<!-- (Update "[ ]" to "[x]" to check a box) -->
+
+**What kind of change does this PR introduce?** (check at least one)
+
+- [ ] Bugfix
+- [ ] Feature
+- [ ] Code style update
+- [ ] Refactor
+- [ ] Build-related changes
+- [ ] Other, please describe:
+
+**Does this PR introduce a breaking change?** (check one)
+
+- [ ] Yes
+- [ ] No
+
+If yes, please describe the impact and migration path for existing applications:
+
+**The PR fulfills these requirements:**
+
+- [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. `fix #xxx[,#xxx]`, where "xxx" is the issue number)
+
+If adding a **new feature**, the PR's description includes:
+- [ ] A convincing reason for adding this feature (it's best to open a suggestion issue first and wait for approval before working on it)
+
+**Other information:**