improve PR template

Signed-off-by: Anton Kosyakov <anton.kosyakov@typefox.io>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1 +1,30 @@
-<!-- Please provide a clear and meaningful description to the CHANGELOG.md file if this PR contributes some significant changes -->
+<!--
+Thank you for your Pull Request. Please provide a description and review
+the requirements below.
+
+Contributors guide: https://github.com/theia-ide/theia/blob/master/CONTRIBUTING.md
+-->
+
+#### What it does
+<!-- Include relevant issues and describe how they are addressed. -->
+
+#### How to test
+<!-- Explain how a reviewer can reproduce a bug, test new functionality or verify perf improvements.  -->
+
+#### Review Checklist
+<!-- For completed items, change [ ] to [x].  -->
+<!-- For inapplicable items, cross out with `[ ] foo` to `[x] ~~foo~~`.  -->
+
+- [ ] as an author I have carefully reviewed and thoroughly tested my changes.
+- [ ] commits are signed-off: https://github.com/theia-ide/theia/blob/master/CONTRIBUTING.md#sign-your-work
+- [ ] commit history is rebased on master and contains only meaningful commits and changes (less are usually better)
+  - for example use `git pull -r` or `git fetch && git rebase` to pick up changes from the master
+- [ ] [changelog](https://github.com/theia-ide/theia/blob/master/CHANGELOG.md) is updated
+- [ ] breaking changes are justified and recorded in the changelog
+- [ ] each new file has a proper copyright with the current year and the name of contributing entity (individual or company)
+- [ ] new dependendencies are justified and [verified](https://github.com/theia-ide/theia/wiki/Registering-CQs#wip---new-ecd-theia-intellectual-property-clearance-approach-experimental)
+- [ ] copied code is justified and [approved via a CQ](https://github.com/theia-ide/theia/wiki/Registering-CQs#case-3rd-party-project-code-copiedforked-from-another-project-into-eclipse-theia-maintained-by-us)
+- [ ] new code is aligned with [project organization](https://github.com/theia-ide/theia/wiki/Code-Organization) and [coding conventions](https://github.com/theia-ide/theia/wiki/Coding-Guidelines)
+- [ ] each review approve is provided with a comment regarding whether functionallity is tested, design or code are aligned with project organization and conventions
+  - an approve without comment should be dismissed
+  - a reviewer should focus on requirements and design validation first, testing second and checking code alignment as the last