DOC: add and clarify guidelines for referencing external code

docs/contributing.md

@@ -25,10 +25,47 @@ contributor is unable to, but this is a time-consuming process and may delay the
 merging of the PR significantly. Please be aware of this when assigning
 yourself to an issue for such algorithms.
 
-When using code from another package or writing code inspired from another implementation,
-please mention this in your PR. At the very least credit must be given where
-applicable. If the package has a different license, using the code as is may not be
-acceptable. Using others code without credit will like result in your PR being closed.
+### Referencing External Code and Implementations
+
+When using code from another package or writing code inspired by another implementation,
+you must clearly acknowledge this in your Pull Request (PR). At a minimum, appropriate
+credit must be given where applicable. If the source package uses a different license,
+using the code verbatim may not be acceptable. Using others’ code without proper credit
+will likely result in your PR being closed.
+
+#### Referencing External Code and Implementations
+
+To ensure transparency, reproducibility, and license compliance, contributors must
+clearly document the use of externally written code, algorithms, or implementations.
+This applies whether code is directly reused, adapted, or inspired by prior work.
+
+##### When attribution is required
+Attribution **must** be provided if:
+- Code is copied or adapted from another open-source project
+- An algorithm is implemented based on a specific paper, repository, or library
+- Implementation details closely follow an existing reference implementation
+
+##### Where to include references
+Depending on the contribution, references should be included in one or more of the following:
+
+- **Docstrings**
+  Include a short `References` section describing the source.
+- **Module or file headers**
+  Required when substantial portions of code are reused or adapted.
+- **User documentation (examples / API docs)**
+  When external work is relevant for understanding usage or behavior.
+- **Pull Request description**
+  Always mention any external sources used during development.
+
+##### Recommended reference format
+Use a clear and consistent format, for example:
+```text
+References
+----------
+[1] Author(s). Title of work.
+    URL or DOI
+```
+
 
 In the following we will give a brief overview of how to contribute to `aeon`. Making
 contributions to open-source projects takes a bit of proactivity and can be daunting at