Add RETURNING to Delete statement.

Add DELETE statement wiki page.

wiki/DELETE.md

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+DELETE statement deletes rows that satisfy the WHERE clause from the specified table. More about delete statement
+in PostgreSQL: https://www.postgresql.org/docs/11/sql-delete.html
+
+Following clauses are supported:
+- WHERE(delete_condition) - Only rows for which delete condition returns true will be deleted.
+- RETURNING(output_expression...) - An expressions to be computed and returned by the DELETE command after each row is deleted. 
+The expression can use any column names of the table. Write _TableName_.AllColumns to return all columns.
+
+### Example
+
+```
+// delete all links with name 'Gmail' and 'Outlook'
+deleteStmt := Link.
+    DELETE().
+    WHERE(Link.Name.IN(String("Gmail"), String("Outlook")))
+```
+
+Debug sql of above statement:
+
+```sql
+DELETE FROM test_sample.link      -- test_sample is name of the schema
+WHERE link.name IN ('Gmail', 'Outlook');
+```
+
+### Execute statement
+
+To execute delete statement and get sql.Result:
+
+```
+res, err := deleteStmt.Exec(db)
+```
+
+To execute delete statement and return records deleted, 
+delete statement has to have RETURNING clause:
+
+```
+deleteStmt := Link.
+    DELETE().
+    WHERE(Link.Name.IN(String("Gmail"), String("Outlook"))).
+    RETURNING(Link.AllColumns)
+    
+dest := []model.Link{}
+
+err := deleteStmt.Query(db, &dest)
+    
+```
+
+Use `ExecContext` and `QueryContext` to provide context object to execution.
+
+##### SQL table used for the example:
+```sql
+CREATE TABLE IF NOT EXISTS link (
+    id serial PRIMARY KEY,
+    url VARCHAR (255) NOT NULL,
+    name VARCHAR (255) NOT NULL,
+    description VARCHAR (255)
+);
+```