message: Update CONTRIBUTING.md
  Description of the extra_filters chart parameter, usage ideas and links to source files.
diff: @@ -1371,6 +1371,7 @@ Note not all fields are correctly catagorized. The fields vary based on visualiz
  | Field | Type | Notes |
  | ------------------------------------------------------------------------------------------------------ | ------------------------------------------------- | ------------------------------------------------- |
  | `adhoc_filters` | _array(object)_ | The **Filters** widget |
  +| `extra_filters` | _array(object)_ | Another pathway to the **Filters** widget.<br/>It is generally used to pass dashboard filter parameters to a chart.<br/>It can be used for appending additional filters to a chart that has been saved with its own filters on an ad-hoc basis if the chart is being used as a standalone widget.<br/><br/>For implementation examples see : [utils test.py](https://github.com/apache/superset/blob/66a4c94a1ed542e69fe6399bab4c01d4540486cf/tests/utils_tests.py#L181)<br/>For insight into how superset processes the contents of this parameter see: [exploreUtils/index.js](https://github.com/apache/superset/blob/93c7f5bb446ec6895d7702835f3157426955d5a9/superset-frontend/src/explore/exploreUtils/index.js#L159) |
  | `columns` | _array(string)_ | The **Breakdowns** widget |
  | `groupby` | _array(string)_ | The **Group by** or **Series** widget |
  | `limit` | _number_ | The **Series Limit** widget |
  @@ -1417,7 +1418,6 @@ Note the `y_axis_format` is defined under various section for some charts.
  | `default_filters` | _N/A_ | |
  | `entity` | _N/A_ | |
  | `expanded_slices` | _N/A_ | |
  -| `extra_filters` | _N/A_ | |
  | `filter_immune_slice_fields` | _N/A_ | |
  | `filter_immune_slices` | _N/A_ | |
  | `flt_col_0` | _N/A_ | |
  