message: add a bit of doc
diff: +//! BIP39 mnemonics
  +//!
  +//! Can be used to generate the root key of a given HDWallet,
  +//! an address or simply convert bits to mnemonic for human friendly
  +//! value.
  +//!
  +//! For more details about the protocol, see Bitcoin Improvment Proposal 39.
  +//!
  +//! # Examples
  +//!
  +//! ```
  +//! extern crate rand;
  +//! extern crate wallet_crypto;
  +//! use wallet_crypto::bip39::{Entropy, MnemonicString, Type::{*}, dictionary};
  +//! use rand::{random};
  +//!
  +//! let entropy = Entropy::generate(Type12Words, || random());
  +//!
  +//! let mnemonic_phrase = entropy.to_mnemonics().to_string(&dictionary::ENGLISH);
  +//! ```
  +
  use rcw::hmac::{Hmac};
  use rcw::sha2::{Sha512};
  use rcw::pbkdf2::{pbkdf2};
  