Add LOCK statement wiki.

wiki/LOCK.md

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+LOCK statement obtains a table-level lock, waiting if necessary for any conflicting locks to be released.
+More about LOCK statement in PostgreSQL: https://www.postgresql.org/docs/11/sql-lock.html  
+
+Following clauses are supported:
+- IN(mode) - mode specifies which locks this lock conflicts with.  
+Mode can be: 
+    - jet.LOCK_ACCESS_SHARE          
+    - jet.LOCK_ROW_SHARE             
+    - jet.LOCK_ROW_EXCLUSIVE         
+    - jet.LOCK_SHARE_UPDATE_EXCLUSIVE
+    - jet.LOCK_SHARE                 
+    - jet.LOCK_SHARE_ROW_EXCLUSIVE   
+    - jet.LOCK_EXCLUSIVE             
+    - jet.LOCK_ACCESS_EXCLUSIVE      
+- NOWAIT() - locked table should not wait for any conflicting locks to be released. If the specified lock(s) 
+cannot be acquired immediately without waiting, the transaction is aborted.
+
+## Example
+
+
+```
+lockStmt := Address.
+        LOCK().
+        IN(jet.LOCK_ACCESS_SHARE).
+        NOWAIT()
+```
+
+Debug SQL of above statement:
+
+```sql
+LOCK TABLE dvds.address IN ACCESS SHARE MODE NOWAIT;
+```
+
+### Execute statement
+
+To execute update statement and get sql.Result:
+
+```
+res, err := lockStmt.Exec(db)
+```
+
+Use `ExecContext` to provide context object to execution.
+
+