message: Add example for getting a value by its column index
diff: @@ -292,6 +292,19 @@ let messages: Vec<CResult<Message>> = rows
  ```
  +or by column position:
  +
  +```rust
  +let messages: Vec<CResult<Message>> = rows
  + .iter()
  + .map(|row| Message {
  + author: row.r_by_index(0).unwrap(),
  + text: row.r_by_index(1).unwrap(),
  + optional_field: row.get_by_index(2)
  + })
  + .collect();
  +```
  +
  There is no difference between Cassandra's List and Sets in terms of Rust.
  They could be represented as `Vec<T>`. To convert a frame into a structure
  that contains a collection of elements do as follows:
  