Add CDB_EstimateRowCount function

See #295

doc/CDB_EstimateRowCount.md

@@ -0,0 +1,25 @@
+Estimate the number of rows of a query.
+
+
+#### Using the function
+
+```sql
+SELECT CDB_EstimateRowCount($$
+  UPDATE addresses SET the_geom = cdb_geocode_street_point(addr, city, state, 'US');
+$$) AS row_count;
+```
+
+Result:
+
+```
+ row_count
+-----------
+         5
+(1 row)
+```
+
+#### Arguments
+
+CDB_EstimateRowCount(query)
+
+* **query** text: the SQL query to estimate the row count for.