Update Pull Request Template (JetBrains#1271)

- Merge Proposed Changes and Issues fixed
- Move it to the top
- Describe `Testing` in more detail
- Add `Release Notes` for the changelog automation.

The same template will be in the main repo and in Skiko (except the CLA
section)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,16 +1,43 @@
-## Proposed Changes
+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
 
-Test: Describe how you tested your changes. Note that this line (with `Test:`) is required, your PR will not build without it!
+<!-- Optional -->
+This should be tested by QA
 
-## Issues Fixed
+## Release Notes
+<!--
+Optional, if omitted - won't be included in the changelog
 
-Fixes: [Optional] The bug on [https://issuetracker.google.com](https://issuetracker.google.com) being fixed
+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
 
 ## Google CLA
 You need to sign the Google Contributor’s License Agreement at https://cla.developers.google.com/.