Add instructions for using test PyPI

Author: tswast

release-procedure.md

@@ -6,10 +6,20 @@
 
         git push pandas-gbq master --tags
 
+*  Build the package
+
+       twine upload dist/*
+
+*  Upload to test PyPI
+
+       twine upload --repository testpypi dist/*
+
+* Try out test PyPI package
+
+       pip install --upgrade --index-url https://test.pypi.org/simple/ --extra-index-url https://pypi.org/simple pandas-gbq
+
 *  Upload to PyPI
 
-        git clean -xfd
-        python setup.py register sdist bdist_wheel --universal
         twine upload dist/*
 
 *  Do a pull-request to the feedstock on `pandas-gbq-feedstock <https://github.com/conda-forge/pandas-gbq-feedstock/>`__