Add a readme description of the gear export

README.md

@@ -104,6 +104,14 @@ Supported export formats:
       change at any time). Only included since it *may* contain additional data
       that could be useful for developers of analysis tools.</sub>
 
+  -   `gear`: gear used in an activity (JSON list).
+
+      <sub>Provides a list of the gear that's added to an activity (such as tracking
+      road versus mountain versus gravel bikes or different pairs of running shoes).
+      Seems to lack a formal schema and should not be counted on as a stable data
+      format (may change at any time). Only included since it *may* contain additional
+      data that could be useful for developers of analysis tools.</sub>
+
 All files are written to the same directory (`activities/` by default).  Each
 activity file is prefixed by its upload timestamp and its activity id.