List data changes from cmu-delphi/covidcast-indicators#47 in a changelog

[krivard]

• Creates a new page to hold the changelog
• Prototypes a changelog format that balances completeness with concision
• Adds references to changelog in each source section of _signals.md

We may want to pick a different anchor date, particularly for Quidel and Google Surveys.

[capnrefsmmat]

I think this looks like a good structure -- just one suggestion to make heading linking easier so it's easy for us to update the changelog as needed

[capnrefsmmat]

In theory, this heading already has an anchor with id 3-june-2020-v1.3 or something like that.

I think, though, that it would be easier for each source section in covidcast_signals.md to link to the corresponding section in covidcast_changelog.md, rather than the section for the most recent update. So you'd only have to link to covidcast_changelog.md#fb-survey, where the user would immediately see the latest changes. That would make updates easier.

[krivard]

Done.