Updates release documentation

- Links both Travis and GitHub CI build systems
- Updates the PyPI release process (handled by the CI)

doc/source/contribute.rst

@@ -55,8 +55,8 @@ At this point, wait for reviewer approval and conclude any discussion that arise
 - Merge the release branch to the ``master`` branch.
 - Also merge the release branch to the ``develop`` branch.
 - Tag the release commit in ``master``, with tag ``vYYYY.MM.DD``. Include a short summary of the changes.
-- Create the release distributions: ``python3 setup.py sdist bdist_wheel``
-- Upload the release to pypi: ``python3 -m twine upload``.
+- Release distributions and PyPI upload should be `handled by the CI
+  <https://github.com/kivy/python-for-android/blob/v2020.04.29/.travis.yml#L60-L70>`_.
 - Add to the Github release page (see e.g. `this example <https://github.com/kivy/python-for-android/releases/tag/v2019.06.06>`__):
   - The python-for-android README summary
   - A short list of major changes in this release, if any
@@ -68,11 +68,13 @@ At this point, wait for reviewer approval and conclude any discussion that arise
 Release checklist
 ~~~~~~~~~~~~~~~~~
 
-- [ ] Check that the [build is passing](https://travis-ci.org/kivy/python-for-android)
+- [ ] Check that the builds are passing
+  - [ ] [Travis build](https://travis-ci.org/kivy/python-for-android)
+  - [ ] [GitHub Action](https://github.com/kivy/python-for-android/actions)
 - [ ] Run the tests locally via `tox`: this performs some long-running tests that are skipped on Travis.
 - [ ] Build and run the [on_device_unit_tests](https://github.com/kivy/python-for-android/tree/master/testapps/on_device_unit_tests) app using buildozer. Check that they all pass.
 - [ ] Build and run the following [testapps](https://github.com/kivy/python-for-android/tree/master/testapps) for arch `armeabi-v7a` and `arm64-v8a`:
-  - `python3 setup_testapp_python3_sqlite_openssl.py apk`
+  - [ ] `python3 setup_testapp_python3_sqlite_openssl.py apk`
     - [ ] `armeabi-v7a`
     - [ ] `arm64-v8a`
 - [ ] Check that the version number is correct