Merge pull request #1835 from passagemath/CONTRIBUTING-improvements

CONTRIBUTING.md: Expand and clarify

Author: mkoeppe

CONTRIBUTING.md

@@ -4,20 +4,22 @@ passagemath is a compatible fork of SageMath.
 
 We merge new releases of SageMath into passagemath on a regular basis.
 
-## Direct code contributions to passagemath (recommended)
+## Direct code/documentation contributions to passagemath (recommended)
 
 On 2025-09-01, the passagemath project was opened to general
 code contributions.  Submit pull requests to the [main passagemath
 repository](https://github.com/passagemath/passagemath) or other
 repositories in the [passagemath GitHub organization](https://github.com/passagemath).
 
 PRs will be reviewed by the passagemath project's maintainer.
+Members of the passagemath organization and the general public are invited to
+contribute to reviewing PRs.
 
 All participation in passagemath is subject to the Code of Conduct.
 Code contributions to passagemath are limited to those that
-support the goals of the fork outlined in [README.md](README.md).
+do not conflict with the goals of the fork outlined in [README.md](README.md).
 
-## Code contributions to upstream SageMath (caution advised)
+## Code/documentation contributions to upstream SageMath (caution advised)
 
 As we merge new releases of SageMath into passagemath on a regular basis,
 it is also possible to contribute to passagemath indirectly, by opening
@@ -49,9 +51,59 @@ are a basis for such efforts.
 - Use `git cherry-pick` or `git rebase --onto` to prepare a branch
   backported to the SageMath `develop` branch.
 
+## Contributing to passagemath by backporting SageMath PRs
+
+Although we already merge new releases of SageMath into passagemath on a
+regular basis, earlier backports of selected changes made in the SageMath
+project are welcome.
+
+As passagemath makes frequent stable releases, in particular bug fixes
+in the Sage library, updates of packages in the Sage distribution, and
+portability improvements are good candidates for backporting to
+passagemath.
+
+- Such PRs are best opened with a title in the form
+  "ORIGINALTITLE by @ORIGINALAUTHOR, backported" or
+  "ORIGINALTITLE by @ORIGINALAUTHOR, rebased".
+  This ensures that the correct attribution is shown in the automatically
+  generated release notes in https://github.com/passagemath/passagemath/releases
+
+- Please include a link to the original PR in the description.
+  There is no need to copy-paste the original description.
+
+- Use `git cherry-pick` or `git rebase --onto` to prepare a branch
+  backported to the passagemath `main` branch.
+
 ## Non-code contributions to passagemath
 
 There are many other ways to contribute to the passagemath project.
 
 For example, help popularize the project by sharing project announcements
 in mailing lists and on social media.
+
+The passagemath project does not accept financial sponsorship.
+
+## Contributing to the larger mathematical software ecosystem
+
+The passagemath project is a major integrating force in the
+[mathematical software landscape](https://github.com/passagemath#passagemath-in-the-mathematical-software-landscape).
+
+passagemath depends on numerous open-source mathematical software
+packages.  Contributing to the development and maintenance of
+dependencies is a great way to contribute to passagemath, too.
+
+Publishing your own packages that depend on passagemath is also a
+wonderful way to contribute to the passagemath project and the
+mathematical software ecosystem.
+
+You are also invited to help with curation activities for mathematical
+software.
+
+- For user packages that depend on the Sage library, curation
+  activities are coordinated in
+  [passagemath/passagemath#248](https://github.com/passagemath/passagemath/issues/248).
+
+- Regarding other mathematical software, we invite the public to open
+  an Issue at https://github.com/passagemath/passagemath/issues to
+  describe the scope of the curation and initial examples of software
+  to be surveyed.