Doc. for new join types

doc/sql.extensions/README.joins.txt

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+--------------
+New JOIN types
+--------------
+
+Author:
+    Adriano dos Santos Fernandes <adrianosf@uol.com.br>
+
+Format:
+	<named columns join> ::=
+        <table reference> <join type> JOIN <table reference> USING ( <column list> )
+
+    <natural join> ::=
+        <table reference> NATURAL <join type> JOIN <table primary>
+
+Notes:
+    Named columns join:
+        1) All columns specified in <column list> should exist in the tables of both sides.
+        2) An equi-join (<left table>.<column> = <right table>.<column>) is automatically
+           created for all columns (ANDed).
+        3) The USING columns can be accessed without qualifier, and in this case, result of
+           it is COALESCE(<left table>.<column>, <right table>.<column>).
+        4) In "SELECT *", USING columns are expanded once, using the above rule.
+
+    Natural join:
+        1) A "named columns join" is automatically created with all columns common to the
+           left and right tables.
+        2) If the is no common column, a CROSS JOIN is created.
+
+Examples:
+    1) select *
+           from employee
+           join department
+               using (dept_no);
+
+    2) select *
+           from employee_project
+           natural join employee
+           natural join project;