Update Pull Request Template (#4698)

Introduce the same template used in [core
repo](https://github.com/JetBrains/compose-multiplatform-core/pull/1271)
(except the CLA section)

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,40 @@
+Describe proposed changes and the issue being fixed
+
+<!-- Optional -->
+Fixes [link to the issue]
+
+## Testing
+<!-- Optional -->
+Describe how you tested your changes. If possible and needed:
+- Test it on a sample project
+- Write unit tests
+- Provide a code snippet
+
+<!-- Optional -->
+This should be tested by QA
+
+## Release Notes
+<!--
+Optional, if omitted - won't be included in the changelog
+
+Sections:
+- Highlights
+- Known issues
+- Breaking changes
+- Features
+- Fixes
+
+Subsections:
+- Multiple Platforms
+- iOS
+- Desktop
+- Web
+- Resources
+- Gradle Plugin
+-->
+> Section - Subsection
+- Describe a change for adding it to https://github.com/JetBrains/compose-multiplatform/blob/master/CHANGELOG.md
+- _(prerelease fix)_ Fix some bug that introduced in a prerelease version (dev/beta). It will be included in a dev/beta changelog, but excluded from a stable changelog
+
+> Section - Subsection
+- Describe another change if needed