Add INSERT wiki page.

wiki/INSERT.md

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+The PostgreSQL INSERT statement is used to insert a single record or multiple records 
+into a table. More about PostgreSQL INSERT statement can be found here: https://www.postgresql.org/docs/11/sql-insert.html
+
+Following SQL table is used for insert 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)
+);
+```
+
+### Insert row by row
+
+```
+insertQuery := Link.INSERT(Link.ID, Link.URL, Link.Name, Link.Description).
+    VALUES(100, "http://www.postgresqltutorial.com", "PostgreSQL Tutorial", DEFAULT).
+    VALUES(101, "http://www.google.com", "Google", DEFAULT).
+    VALUES(102, "http://www.yahoo.com", "Yahoo", nil)
+```
+
+There is also shorthand notation for inserting model data:
+```
+tutorial := model.Link{
+    ID:   100,
+    URL:  "http://www.postgresqltutorial.com",
+    Name: "PostgreSQL Tutorial",
+}
+
+google := model.Link{
+    ID:   101,
+    URL:  "http://www.google.com",
+    Name: "Google",
+}
+
+yahoo := model.Link{
+    ID:   102,
+    URL:  "http://www.yahoo.com",
+    Name: "Yahoo",
+}
+
+insertQuery := Link.INSERT(Link.ID, Link.URL, Link.Name, Link.Description).
+    MODEL(turorial).
+    MODEL(google).
+    MODEL(yahoo)
+    
+// Or event shorter if model data is in the slice:
+insertQuery := Link.INSERT(Link.ID, Link.URL, Link.Name, Link.Description).
+    MODELS([]model.Link{turorial, google, yahoo})
+```
+`Link.ID, Link.URL, Link.Name, Link.Description` - is the same as Link.AllColumns 
+so above statement can be simplified to:
+
+```
+insertQuery := Link.INSERT(Link.AllColumns).
+    MODELS([]model.Link{turorial, google, yahoo})
+```
+
+`Link.ID` is a primary key autoincrement column so it can be omitted in INSERT statement.  
+`Link.MutableColumns` - is shorthand notation for list of all columns minus primary key columns.
+
+```
+insertQuery := Link.INSERT(Link.MutableColumns).
+    MODELS([]model.Link{turorial, google, yahoo})
+```
+
+Inserts using `VALUES`, `MODEL` and `MODELS` can appear as the part of the same insert statement.
+
+```
+insertQuery := Link.INSERT(Link.ID, Link.URL, Link.Name, Link.Description, Link.Description).
+    VALUES(101, "http://www.google.com", "Google", DEFAULT, DEFAULT).
+    MODEL(turorial).
+    MODELS([]model.Link{yahoo})
+``` 
+
+### Insert using query
+A query (SELECT statement) that supplies the rows to be inserted.
+```
+// duplicate first 10 entries
+query := Link.
+    INSERT(Link.URL, Link.Name).
+    QUERY(
+        SELECT(Link.URL, Link.Name).
+            FROM(Link).
+            WHERE(Link.ID.GT(Int(0)).AND(Link.ID.LT_EQ(10))),
+    )
+```