Show good covidcast/delphi.epidata license usage in vignettes

[brookslogan]

We should make sure that the vignettes serve as a model for other users as to how to apply the licenses of covidcast/delphi.epidata data sources. Interacts with #72 (we may do things differently for a query vs. for internal data vs. for both).

[brookslogan]

Slack thread here.

For CC BY 4.0 data sources (we should check to make sure we only use these or consider other licenses separately), it looks like:

• we can keep the package LICENSE as MIT
• we should include an appropriate "attribution" in the documentation for any associated data package members
• in vignettes using such data, we should also include such an "attribution" (which should be visible in the rendered vignette, not just source), which might look a little different, e.g., in the notes about modifications section

[dajmcdon]

@ChloeYou See the comment above. As you work through #72, can you be sure to address the bullets?

[brookslogan]

This issue was addressed as part of #77, but some issues remain, noted in #84.