Update .github/PULL_REQUEST_TEMPLATE.md

Author: Rich-Harris

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,6 +2,9 @@
 
 Please note that [the Svelte codebase is currently being rewritten for Svelte 5](https://svelte.dev/blog/runes). Changes should target Svelte 5, which lives on the default branch (`main`).
 
+If your PR concerns Svelte 4 (including updates to [svelte.dev.docs](https://svelte.dev/docs)), please ensure the base branch is `svelte-4` and not `main`.
+
+
 ### 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