update readme

[dfarrow0]

• make covidcast more prominent
• trim down other content
• move references to new page

[dfarrow0]

note this got stacked on top of my other documentation PR, #252 -- suggest merging that one first.

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@capnrefsmmat @ryantibs please take a look

[ryantibs]

Looks good, thank you @dfarrow0. I left two very small comments.

Since this builds off your other doc update #252, I suggest @krivard (who is the reviewer for that one) merges this one, after she merges that one.

[dfarrow0]

Thanks Ryan, made the edits you suggested.

@krivard merge when ready