Add merged-point-visualization docs.

README.md

@@ -80,6 +80,7 @@ Each rule within the YAML `rules` array can have the following fields:
 | `flat`                        | Clamps the feature to the ground (Does not work for meshes).                                                                                                                                                | Boolean                                                    | `true`, `false`                      |
 | `outline-color`               | Point outline color.                                                                                                                                                                                        | String                                                     | `green`, `#fff`                      |
 | `outline-width`               | Point outline width in px.                                                                                                                                                                                  | Float                                                      | `3.6`                                |
+| `point-merge-grid-cell`       | WGS84/altutide meter tolerance for merging point visualizations.                                                                                                                                            | Array of three Floats.                                     | `[0.000000084, 0.000000084, 0.01]`   |
 | `near-far-scale`              | For points, indicate (`near-alt-meters`, `near-scale`, `far-alt-meters`, `far-scale`).                                                                                                                      | Array of four Floats.                                      | `[1.5e2,10,8.0e6,0]`                 |
 | `offset`                      | Apply a fixed offset to each shape-point in meters. Can be used for z-ordering.                                                                                                                             | Array of three Floats.                                     | `[0, 0, 5]`                          |
 | `arrow`                       | For arrow-heads: One of `none`, `forward`, `backward`, `double`. Not compatible with `dashed`.                                                                                                              | String                                                     | `single`                             |
@@ -230,6 +231,17 @@ For attributes, style expressions (e.g. `color-expression`) are evaluated in a c
 set the `offset` field. The spatial `offset` will be multiplied, so it is possible to "stack" attributes
 over a feature.
 
+### About Merged Point Visualizations
+
+By setting `point-merge-grid-cell`, a tolerance may be defined which allows merging the visual representations
+of point features which share the same 3D spatial cell, map, layer, and style rule. This has two advantages:
+
+* **Multi-Selection**: When selecting the merged representation, a multi-selection of all merged features happens.
+* **Logical Evaluation using `$mergeCount`**: In some map formats, it may be desirable to apply a style based on the number of merged points.
+  This may be done to display a warning, or to check a matching requirement.
+  To this end, the `$mergeCount` variable is injected into each simfil evaluation context of a merged-point style rule.
+  Check out the default style for an example.
+
 ### About `first-of`
 
 Normally, all style rules from a style sheet are naively applied to all matching features.