Add contributing doc

Author: wboykinm

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+## Publish to pypi
+
+Following the [general guidelines for publishing](https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/), and using Mapbox pypi account credentials, we can make new versions of `rio-color` available on pypi. This approach replaces our previous use of TravisCI and Appveyor, both now deprecated with our organization.
+
+### Build packages
+
+```
+python3 -m pip install build twine
+python3 -m build --sdist
+python3 -m build --wheel
+```
+
+### Upload packages
+Once you've configured `.pypirc` [with the project token](https://pypi.org/help/#apitoken), you can upload the packages to pypi.
+
+```
+twine upload dist/* --repository rio-color
+```
+
+And confirm the upload at [the project page](https://pypi.org/project/rio-color/#history).