List geo changes to safegraph in changelog

Author: krivard

docs/api/covidcast-signals/safegraph.md

@@ -8,8 +8,8 @@ grand_parent: COVIDcast Epidata API
 
 * **Source name:** `safegraph`
 * **First issued:** 23 June 2020
-* **Number of data revisions since 23 June 2020:** 0
-* **Date of last change:** never
+* **Number of data revisions since 23 June 2020:** 1
+* **Date of last change:** 3 November 2020
 * **Available for:** county, state (see [geography coding docs](../covidcast_geography.md))
 
 This data source uses data reported by [SafeGraph](https://www.safegraph.com/)

docs/api/covidcast_changelog.md

@@ -86,6 +86,11 @@ The 10 October 2020 issue of all `indicator-combination` deaths signals has been
 #### 22 October 2020
 We went from a custom geo mapping file (for aggregating from zip->(county, msa, hrr, state)) to a central geo file based on rigorously sourced US census data.
 
+### `safegraph`
+
+#### 3 November 2020
+We went from a custom geo mapping file (for aggregating from zip->(county, msa, hrr, state)) to a central geo file based on rigorously sourced US census data.
+
 ### `usa-facts`
 
 #### 12 October 2020