message: Add connection pool section
diff: @@ -128,6 +128,36 @@ let ssl_transport = Transport::new(addr, &connector).unwrap();
  let client = CDRS::new(ssl_transport, authenticator);
  ```
  +### Connecting via r2d2 connection pool
  +
  +There is an option to create [r2d2](https://github.com/sfackler/r2d2) connection pool
  +of CDRS connections both plain and SSL-encrypted:
  +
  +```rust
  +use cdrs::connection_manager::ConnectionManager;
  +
  +let config = r2d2::Config::builder()
  + .pool_size(15)
  + .build();
  +let transport = Transport::new(ADDR).unwrap();
  +let authenticator = PasswordAuthenticator::new(USER, PASS);
  +let manager = ConnectionManager::new(transport, authenticator, Compression::None);
  +
  +let pool = r2d2::Pool::new(config, manager).unwrap();
  +
  +for _ in 0..20 {
  + let pool = pool.clone();
  + thread::spawn(move || {
  + let conn = pool.get().unwrap();
  + // use the connection
  + // it will be returned to the pool when it falls out of scope.
  + });
  +}
  +
  +```
  +
  +There is a related example.
  +
  ### Getting supported options
  Before session established an application may want to know which options are
  