chore: update pull request template to include changelog placeholders (#338)

Author: zimeg

.github/pull_request_template.md

@@ -1,8 +1,12 @@
+### Changelog
+
+(Add a note with features or fixes that change the user experience for release notes)
+
 ### Summary
 
 (Please describe the goal of this pull request and mention any related issue numbers)
 
 ### Requirements
 
-* [ ] I've read and understood the [Contributing Guidelines](/blob/main/.github/contributing.md) and have done my best effort to follow them.
-* [ ] I've read and agree to the [Code of Conduct](https://slackhq.github.io/code-of-conduct).
+- [ ] I've read and understood the [Contributing Guidelines](https://github.com/slackapi/slack-cli/blob/main/.github/CONTRIBUTING.md) and have done my best effort to follow them.
+- [ ] I've read and agree to the [Code of Conduct](https://slackhq.github.io/code-of-conduct).