message: add usage doc
diff: @@ -47,3 +47,20 @@ where `K : Vec<u8>` read from the hint file and V is a rust struct `KeyDirEntry`
  | key_size | `u64` | Key size |
  | value_size | `u64` | Value size |
  | data_entry_position | `u64` | Data entry position in File |
  +
  +
  +# Usage
  +
  +```rust
  + let db = EdgeKV::open("db").unwrap();
  +
  + let k = b"k1";
  +
  + db.put(k.to_vec(), vec![0]).unwrap();
  + db.merge(concatenate_merge, k.to_vec(), vec![1]).unwrap();
  + db.merge(concatenate_merge, k.to_vec(), vec![2]).unwrap();
  + assert_eq!(db.get(&k.to_vec()).unwrap().unwrap(), vec![0, 1, 2]);
  +
  + db.put(k.to_vec(), vec![3]).unwrap();
  + assert_eq!(db.get(&k.to_vec()).unwrap().unwrap(), vec![3]);
  +```
  \ No newline at end of file
  