Update PULL_REQUEST_TEMPLATE.md

PULL_REQUEST_TEMPLATE.md

@@ -1,9 +1,22 @@
-First of all, please read [`CONTRIBUTING.md`](https://github.com/mattermost/desktop/blob/master/CONTRIBUTING.md)
+Thank you for planning to submit a pull request and contributing to the Mattermost Desktop application!
 
----
+Before submitting, please confirm you've 
+ - [ ] read and understood our [Contributing Guidelines](https://github.com/mattermost/desktop/blob/master/CONTRIBUTING.md)
+ - [ ] completed [Mattermost Contributor Agreement](http://www.mattermost.org/mattermost-contributor-agreement/)
+ - [ ] executed `npm run prettify` for proper code formatting
 
-- [ ] Complete [Mattermost Contributor Agreement](http://www.mattermost.org/mattermost-contributor-agreement/)
-- [ ] Execute `npm run prettify` to format codes
-- [ ] Update `CHANGELOG.md` and/or `doc/*.md` if it's necessary.
-- [ ] Write about environment which you tested
-  - Operating system
+Please provide the following information:
+
+**Summary**
+<!--
+Write a short (one line) summary that describes the changes in this pull request for inclusion in the changelog
+-->
+
+**Issue link**
+<!--
+Please include a link to the GitHub issue this pull request addresses, if applicable.
+-->
+
+**Test Cases**
+
+**Additional Notes**