add changelog (#218)

add changelog

.github/PULL_REQUEST_TEMPLATE.md

 ## Checklist
 - [ ] Ensured this PR is up-to-date with the `dev` branch
 - [ ] Created this PR to target the `dev` branch
-- [ ] Followed the style guidelines as described in the [Contribution Guidelines](../CONTRIBUTING.md)
+- [ ] Followed the style guidelines as described in the [Contribution Guidelines](https://github.com/Unity-Technologies/Unity-Robotics-Hub/blob/main/CONTRIBUTING.md)
+- [ ] Updated the [Changelog](https://github.com/Unity-Technologies/Unity-Robotics-Hub/blob/dev/CHANGELOG.md) and described changes in the [Unreleased section](https://github.com/Unity-Technologies/Unity-Robotics-Hub/blob/dev/CHANGELOG.md#unreleased)
 - [ ] Updated the documentation as appropriate
 
 ## Other comments