message: Add: wallet object `signMessage` method documentation
diff: @@ -28,6 +28,7 @@ WalletInstance {
  sendWithConfirmation(transactionObject: Object, confirmation: Promise<Boolean> | Boolean): Promise<String>,
  setDefaultAddress(addressIndex: Number): Promise<Boolean>,
  sign(transactionObject: Object): Promise<String>
  + signMessage(messageObject: Object): Promise<String>
  }
  ```
  @@ -58,6 +59,7 @@ _**Example:** Instantiating a software wallet using an existing `privateKey` wil
  * [`sendWithConfirmation()`](#sendwithconfirmation)
  * [`setDefaultAddress()`](#setdefaultaddress)
  * [`sign()`](#sign)
  + * [`signMessage()`](#signmessage)
  ## Props
  @@ -493,3 +495,30 @@ const transaction = {
  const transactionSignature = await trezorWallet.sign(transaction);
  ```
  +
  +### `signMessage()`
  +```js
  +WalletInstance.signMessage(messageObject: Object): Promise<String>
  +```
  +
  +Sign a message with your public key to prove your identity and ownership of the address.
  +
  +This method takes in an `messageObject` Object _(See below)_, and returns the hex `String` signature wrapped inside a `Promise` _(This method is `async`)_.
  +
  +The `messageObject` only has one prop, `message`, but to keep consistency with the rest of the library, it is passed in as an Object.
  +
  +**`messageObject` format:**
  +```js
  +messageObject {
  + message: String // The message you want to sign as a String. Default to the '' empty String
  +}
  +```
  +
  +**Usage:**
  +```js
  +import { open } from 'colony-wallet/hardware/trezor';
  +
  +const trezorWallet = await open();
  +
  +const messageSignature = await trezorWallet.sign({ message: 'Yes, this is me!' });
  +```
  