docs: Add a contributing guide

Added `docs/CONTRIBUTING.md` with some details of how to contribute to
RDFLib.

This file should be picked up by GitHub and satisfy one of the checklist
items under the [Community Standards](https://github.com/RDFLib/rdflib/community)
page.

docs/CONTRIBUTING.md

@@ -0,0 +1,32 @@
+# RDFLib Contributing Guide
+
+Thank you for considering contributing to RDFLib. This project has no formal
+funding or full-time maintainers and relies entirely on independent contributors
+to keep it alive and relevant.
+
+## Ways to contribute
+
+Some ways in which you can contribute to RDFLib are:
+
+- Address open issues: [![GitHub issues](https://img.shields.io/github/issues/RDFLib/rdflib)](https://github.com/RDFLib/rdflib/issues)
+- Fix [expected failure](https://docs.pytest.org/en/latest/how-to/skipping.html) tests.
+- Add additional [expected failure](https://docs.pytest.org/en/latest/how-to/skipping.html) failure tests for open issues: [![GitHub issues](https://img.shields.io/github/issues/RDFLib/rdflib)](https://github.com/RDFLib/rdflib/issues)
+- Add tests for untested code: [![Coveralls branch](https://img.shields.io/coveralls/RDFLib/rdflib/master.svg)](https://coveralls.io/r/RDFLib/rdflib?branch=master)
+- Reviewing pull requests marked with the [![review wanted](https://img.shields.io/badge/-review%20wanted-28ead2)](https://github.com/RDFLib/rdflib/labels/review%20wanted) label.
+- Answer questions on Stack Overflow: [![Stack Exchange questions](https://img.shields.io/stackexchange/stackoverflow/t/rdflib)](https://stackoverflow.com/questions/tagged/rdflib)
+- Convert [`unittest`](https://docs.python.org/3/library/unittest.html) based tests to [`pytest`](https://docs.pytest.org/en/latest/) based tests: [![GitHub search query](https://img.shields.io/badge/GitHub-search-green)](https://github.com/search?q=unittest+repo%3ARDFLib%2Frdflib+extension%3Apy+path%3Atest%2F&type=Code)
+- Add, correct or improve docstrings: [![rtd latest](https://img.shields.io/badge/docs-latest-informational)](https://pynif3d.readthedocs.io/en/latest/)
+- Update the RDFLib Wikipedia entry: [![Wikipedia: RDFLib](https://img.shields.io/badge/Wikipedia-RDFLib-informational)](https://en.wikipedia.org/wiki/RDFLib)
+- Update the RDFLib Wikidata entry: [![Wikidata: Q7276224](https://img.shields.io/badge/Wikidata-Q7276224-informational)](https://www.wikidata.org/wiki/Q7276224)
+- Particpate on Gitter/Matrix chat: [![Gitter](https://badges.gitter.im/RDFLib/rdflib.svg)](https://gitter.im/RDFLib/rdflib?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge) [![Matrix](https://img.shields.io/matrix/rdflib:matrix.org?label=matrix.org%20chat)](https://matrix.to/#/#RDFLib_rdflib:gitter.im)
+- Particpate in GitHub discussions: [![GitHub Discussions](https://img.shields.io/github/discussions/RDFLib/rdflib)](https://github.com/RDFLib/rdflib/discussions)
+- Fix flake8 failures.
+
+## Pull Requests
+
+Contributions that involve changes to the RDFLib repository have to be made with
+pull requests and should follow the [RDFLib developers guide](./developers.md).
+
+## Code of Conduct
+
+All contributions to the project should be consistent with the [code of conduct](./CODE_OF_CONDUCT.md) adopted by RDFLib.

docs/index.rst

@@ -88,6 +88,7 @@ For developers
    docs
    persisting_n3_terms
    type_hints
+   CONTRIBUTING
 
 Source Code
 -----------