Convert contributing guidelines to Markdown

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# How to contribute
+
+Development of nbstripout happens on
+[GitHub](https://github.com/kynan/nbstripout) -
+[bug reports](https://github.com/kynan/nbstripout/issues) and
+[pull requests](https://github.com/kynan/nbstripout/pulls) welcome!
+
+# Releasing a new version
+
+To simplify updating the version number consistently across different files and
+creating the appropriate annotated tag, we use
+[bump-my-version](https://github.com/callowayproject/bump-my-version). For a new
+patch release, run
+
+    bump-my-version bump patch
+
+and for a minor release, run
+
+    bump-my-version bump minor
+
+Remember to also push the release tag with `git push --tags`.
+
+Use [twine](https://twine.readthedocs.io/en/latest/#using-twine) to upload the
+new release to PyPI:
+
+    python -m build
+    twine check dist/nbstripout-0.6.2*
+    twine upload -r testpypi dist/nbstripout-0.6.2*
+    twine upload dist/nbstripout-0.6.2*

setup.cfg

@@ -5,6 +5,8 @@ tag = True
 tag_name = {new_version}
 tag_message = v{new_version}
 
+[bumpversion:file:CONTRIBUTING.md]
+
 [bumpversion:file:README.rst]
 search = rev: {current_version}
 replace = rev: {new_version}