Document FIRMWARE_FORMAT (#3631)

docs/config_options.md

@@ -199,6 +199,8 @@ This is a [make](https://www.gnu.org/software/make/manual/make.html) file that i
 
 * `DEFAULT_FOLDER`
   * Used to specify a default folder when a keyboard has more than one sub-folder.
+* `FIRMWARE_FORMAT`
+  * Defines which format (bin, hex) is copied to the root `qmk_firmware` folder after building.
 * `SRC`
   * Used to add files to the compilation/linking list.
 * `LAYOUTS`