Add UPDATE statement wiki page.

wiki/UPDATE.md

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+UPDATE changes the values of the specified columns in all rows that satisfy the condition.
+More about UPDATE statement in PostgreSQL: https://www.postgresql.org/docs/11/sql-update.html
+
+Following clauses are supported 
+- UPDATE(columns...) - list of columns to update
+- SET(values...) - list of values for columns
+- MODEL(model) - list of values for columns will be extracted from model object
+- WHERE(condition) - row condition to update
+- RETURNING(columns...) - list of columns to return as statement result
+
+_This list might be extended with feature Jet releases._ 
+
+## Example
+
+```
+// replace all Bing links with Yahoo
+updateStmt := Link.
+    UPDATE(Link.Name, Link.URL).
+    SET("Yahoo", "http://yahoo.com").
+    WHERE(Link.Name.EQ(String("Bing")))
+```
+
+Debug sql of above statement:
+```sql
+UPDATE test_sample.link          -- 'test_sample' is name of the schema
+SET (name, url) = ('Bong', 'http://bong.com')
+WHERE link.name = 'Bing';
+```
+
+Short-hand notation to extract model data for column values:
+
+```
+yahoo := model.Link{
+    URL:  "http://www.yahoo.com",
+    Name: "Yahoo",
+}
+
+updateStmt := Link.
+    UPDATE(Link.Name, Link.URL, Link.Description).
+    MODEL(yahoo).
+    WHERE(Link.Name.EQ(String("Bing")))
+```
+
+`Link.Name, Link.URL, Link.Description` - can be replaced with Link.MutableColumns. All columns minus primary key columns.
+Primary key columns are not updated usually.
+
+```
+updateStmt := Link.
+    UPDATE(Link.MutableColumns).
+    MODEL(yahoo).
+    WHERE(Link.Name.EQ(String("Bing")))
+```
+
+### Execute statement
+
+To execute update statement and get sql.Result:
+
+```
+res, err := updateStmt.Exec(db)
+```
+
+To execute update statement and return row records updated, statement has to have RETURNING clause:
+```
+updateStmt := Link.
+    UPDATE(Link.MutableColumns).
+    MODEL(yahoo).
+    WHERE(Link.Name.EQ(String("Bing"))).
+    RETURNING(Link.AllColumns)
+    
+dest := []model.Link{}
+
+err := updateStmt.Query(db, &dest)
+    
+```
+
+Use `ExecContext` and `QueryContext` to provide context object to execution.
+
+Update example SQL table:
+```sql
+CREATE TABLE IF NOT EXISTS link (
+    id serial PRIMARY KEY,
+    url VARCHAR (255) NOT NULL,
+    name VARCHAR (255) NOT NULL,
+    description VARCHAR (255)
+);
+```
+