Create the initial .github/PULL_REQUEST_TEMPLATE.md for this repo (#83)

Now consistent with the Flutter IJ repo. Related PR: flutter/flutter-intellij#8614

Author: jwren

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,14 @@
+- Thanks for your contribution! Please replace this text with a description of what this PR is changing or adding and why, list any relevant issues, and review the contribution guidelines below.
+
+---
+
+- [ ] I’ve reviewed the contributor guide and applied the relevant portions to this PR.
+
+<details>
+  <summary>Contribution guidelines:</summary><br>
+
+- See our [contributor guide]([https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md) for general expectations for PRs.
+- Larger or significant changes should be discussed in an issue before creating a PR.
+- Dart contributions to our repos should follow the [Dart style guide](https://dart.dev/guides/language/effective-dart) and use `dart format`.
+- Java and Kotlin contributions should strive to follow Java and Kotlin best practices ([discussion](https://github.com/flutter/flutter-intellij/issues/8098)).
+</details>