Update contributing guidelines

.github/pull_request_template.md

@@ -2,17 +2,23 @@
 
 <!--
 Thanks for your willingness to contribute to Instaloader! Please briefly
-describe
+describe:
 
 - A motivation for this change, e.g.
   - Fixes # .
-  - More general: What problem does the pull request solve?
+  - What problem does the pull request solve?
 
 - The changes proposed in this pull request
+  - Keep the PR focused: avoid bundling independent fixes or refactorings.
 
 - The completeness of this change
-  - Is it just a proof of concept?
   - Is the documentation updated (if appropriate)?
-  - Do you consider it ready to be merged or is it a draft?
+  - How has it been tested?
+  - Do you consider it ready to be merged or is it a draft / proof of concept?
   - Can we help you at some point?
+
+- Further considerations
+  - Is it based on master (bugfix), upcoming/v4.X (new feature), or another pull request?
+  - Does it preserve backward compatibility and existing behavior?
+
 -->

docs/contributing.rst

@@ -74,6 +74,14 @@ Improvements of the Instaloader source or its documentation can be proposed as a
     - a bug fix that does require thorough testing before being released to a
       final version.
 
+- Keep Pull Requests focused: address a single problem or feature in a
+  self-contained change. Avoid bundling multiple independent fixes, refactorings,
+  or architectural changes into one pull request, as this makes it hard to
+  review, validate, or selectively merge them.
+
+- Be mindful of backward compatibility: changes should preserve existing intended
+  behavior whenever possible.
+
 - All Pull Requests are analyzed by `Pylint <https://www.pylint.org/>`__ for
   error and syntax checking of the source and
   `Mypy <https://github.com/python/mypy>`__ for type checking. You can run them