update symptom survey overview to mention API and contingency aggregates
update symptom survey overview to mention API and contingency aggregates

cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata,cmu-delphi/delphi-epidata