Updating "README.md" to avoid confusion (#1054)

* Updating "README.md" to avoid confusion

#1053 #1030 #973 etc.

* Updating "README.md" based on further discussion regarding avoiding confusion

#1054

* Tweaking language in "README.md" while avoiding confusion

#1054

* Update "README.md" due to link issue from copy/paste

Author: benguild

website/docs/README.md

@@ -13,7 +13,7 @@ The H3 Core Library implements the H3 grid system. It includes functions for con
 
 ## Highlights
 
-* H3 is a hierarchical [geospatial index](/docs/highlights/indexing).
+* H3 is a [hierarchical geospatial index](/docs/highlights/indexing), which provides [exact logical containment but only approximate geometric containment](https://observablehq.com/@nrabinowitz/h3-hierarchical-non-containment) across the cell hierarchy. That is, **points within children may have different parent cells,** because child cells are not always entirely geometrically contained by the same parent.
 * H3 was developed to address the [challenges of Uber's data science needs](/docs/highlights/aggregation).
 * H3 can be used to [join disparate data sets](/docs/highlights/joining).
 * In addition to the benefits of the hexagonal grid shape, H3 includes features for [modeling flow](/docs/highlights/flowmodel).