update PR template

Author: ytdHuang

.github/pull_request_template.md

@@ -1,11 +1,12 @@
 ## Checklist
 Thank you for contributing to `QuantumToolbox.jl`! Please make sure you have finished the following tasks before opening the PR.
 
-- [ ] Please read [Contributing to QuantumToolbox.jl](https://qutip.org/QuantumToolbox.jl/stable/resources/contribute).
+- [ ] Please read [Contributing to QuantumToolbox.jl](https://qutip.org/QuantumToolbox.jl/stable/resources/contributing).
 - [ ] Any code changes were done in a way that does not break public API.
 - [ ] Appropriate tests were added and tested locally by running: `make test`.
 - [ ] Any code changes should be `julia` formatted by running: `make format`.
 - [ ] All documents (in `docs/` folder) related to code changes were updated and able to build locally by running: `make docs`.
+- [ ] (If necessary) the `CHANGELOG.md` should be updated (regarding to the code changes) and built by running: `make changelog`.
 
 Request for a review after you have completed all the tasks. If you have not finished them all, you can also open a [Draft Pull Request](https://github.blog/2019-02-14-introducing-draft-pull-requests/) to let the others know this on-going work.