Update covidcast_changelog.md

Author: Akvannortwick

docs/api/covidcast_changelog.md

@@ -40,6 +40,14 @@ Duplicate survey weights had corrupted historical figures for the following sign
   * `hrr`: 20200406-20200415, 20200430-20200506
   * `msa`: 20200408-20200414, 20200430-20200506
   * `state`: 20200408-20200416, 20200430-20200506
+  
+#### 20 November 2020  
+
+Due to a bug in our data processing system, estimates of the percentage of people reporting that they have been tested for COVID-19 calculated before October 8th were incorrect. 
+We incorrectly treated an answer of “no” as a missing response, which affected the smoothed_tested_14d and smoothed_wtested_14d signals from the fb-survey source. 
+
+As of Nov. 20th, the error has been corrected and all affected data reissued. 
+
 
 ### `google-survey`
 ### `ght`