Slightly improve PR template

Author: mara004

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,8 +7,7 @@
 ## Checklist
 
 <!--
-- Choose the section(s) relevant to your PR and answer the questions. A section is indicated by `### Heading`.
-- Non-applying sections can be ignored or removed.
+- Choose the section(s) relevant to your PR and answer the questions. A section is indicated by `### Heading`. Skip non-applying sections.
 - Place an `x` in the [ ] for yes, leave it empty for no. If a question is not applicable, remove the [ ], but keep the message in place.
 - Use the Preview tab to confirm the PR will render correctly.
 -->
@@ -26,8 +25,13 @@
 ### Setup
 
 - [ ] This PR changes setup code (`setupsrc/`, `setup.py` etc.).
+- [ ] I have read through relevant doc sections [Installation -> From source][1] and [Setup Magic][2].
 - [ ] I have tested the change does not break internal callers.
 - [ ] I believe the change is maintainable and does not cause unreasonable complexity or code pollution.
-- [ ] The change does not try to shift a maintenance burden or upstream downstream tasks. *Keep handlers generic, avoid overly downstream-specific or effectively dead code passages.*
-- [ ] I have assessed that the targeted use case cannot reasonably be satisfied by existing means, or the change forms a notable improvement over possible alternatives.
+- [ ] The change does not try to shift a maintenance burden or upstream downstream tasks. *Keep handlers generic, avoid overly downstream-specific or (for us) effectively dead code passages.*
+- [ ] I have assessed that the targeted use case cannot reasonably be satisfied by existing means, such as [Caller-provided data files][3], or the change forms a notable improvement over possible alternatives.
 - [ ] I believe the targeted use case is supported by pypdfium2-team. *Note that we may not want to support esoteric or artificially restricted setup envs.*
+
+[1]: https://github.com/pypdfium2-team/pypdfium2?tab=readme-ov-file#install-source
+[2]: https://github.com/pypdfium2-team/pypdfium2?tab=readme-ov-file#setup-magic
+[3]: https://github.com/pypdfium2-team/pypdfium2?tab=readme-ov-file#install-source-caller