docs: add CHANGELOG.unreleased.md (#5775)

Ref: https://github.com/prettier/prettier/issues/5735#issuecomment-452784056

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,6 +5,7 @@
 
 - [ ] I’ve added tests to confirm my change works.
 - [ ] (If changing the API or CLI) I’ve documented the changes I’ve made (in the `docs/` directory)
+- [ ] (If not an internal change) I’ve added my changes to the `CHANGELOG.unreleased.md` file following the template.
 - [ ] I’ve read the [contributing guidelines](https://github.com/prettier/prettier/blob/master/CONTRIBUTING.md).
 
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