Add common GitHub Actions recipes

[willbarton]

This is an attempt to codify some common GitHub Actions recipes that we can use, probably with tweaks, across multiple repositories.

It is based in part on work that the Dev Services team did to add GitHub Actions to teachers-digital-platform and previously to cfgov-refresh

[Scotchester]

Workflow templates just announced: https://github.blog/changelog/2020-06-23-github-actions-workflow-templates/

[willbarton]

Workflow templates just announced: https://github.blog/changelog/2020-06-23-github-actions-workflow-templates/

Naturally.

[willbarton]

@Scotchester I've updated this PR (and opened https://github.com/cfpb/.github/pull/1) based on the GitHub Actions workflow templates. It now has screenshots of how to use the templates to hopefully https://github.com/cfpb/.github/pull/1 will provide, as well as the recipes for uploading a wheel file to GitHub and the kind of git checkout that's required for setuptools-git-version (which the templates provide, but I think is worth calling out here separately too).