cdb_union_adjacent documentation

pg/doc/05_cdb_union_adjacent.md

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+### Union Adjacent
+
+This is an aggregate function that will take a set of polygons and return a geometry array
+of regions where the polygons are continuous. Basically it combines polygons
+which are touching in to single polygons.
+
+It takes a single value:
+
+* `geometry` a list of geometries to be clustered and joined
+
+and returns
+
+* `geometry[]` an array of the joined geometries.
+
+An example usage would be something like:
+
+```postgresql
+  with joined_polygons as (
+    select cdb_union_adjacent(the_geom) regions from some_table
+  )
+  select unnest(region) the_geom from joined_polygons
+```
+
+which will produce a table with regions of continuous polygons from the original
+table.