Sketch for new Overview-creation functionality

doc/CDB_Overviews.md

@@ -0,0 +1,28 @@
+Create overviews for vector dataset.
+
+#### Using the function
+
+The table for which overviews will be generated should be
+a Cartodbfied dataset with vector geometry.
+
+```sql
+SELECT CDB_CreateOverviews('table_name');
+--- Generates overview tables for the dataset
+```
+
+#### Arguments
+
+CDB_CreateOverviews(table_name, ref_z_strategy, reduction_strategy)
+
+* **table_name** regclass, table for which overviews will be generated
+* **ref_z_strategy** regproc, function that provides the Z-scale strategy.
+  It returns the base Z level for the dataset.
+  It should have these arguments:
+  - **table_name** regclass, table to compute the reference Z scale for
+* **reduction_strategy** regproc, function that provides the reduction strategy
+  to generate an overview table from a table for a smaller scale (higher Z number).
+  It returns the name of the generated table.
+  It should have these arguments:
+  - **base_table_name** regclass, base table to be reduced.
+  - **base_z** integer, base Z level assigned to the base table.
+  - **overview_z** integer, Z level for which to generate the overview.