Update PR template to ask for PR category (#6475)

Author: benmccann

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,6 @@
 ### Before submitting the PR, please make sure you do the following
 - [ ] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
+- [ ] Prefix your PR title with `[feat]`, `[fix]`, `[chore]`, or `[docs]`.
 - [ ] This message body should clearly illustrate what problems it solves.
 - [ ] Ideally, include a test that fails without this PR but passes with it.