message: Add: purser-metamask `accountChangeHook` docs
diff: @@ -91,9 +91,36 @@ This method returns a `Promise` which, after resolving, it will `return` only re
  **Usage examples:**
  -Open the metamask wallet:
  +Detect if Metamask is available:
  ```js
  import { detect as isMetamaskAvailable } from '@colony/purser-metamask';
  await isMetamaskAvailable(); // true
  ```
  +
  +### `accountChangeHook`
  +
  +```js
  +await accountChangeHook(callback: Function);
  +```
  +
  +This is a utility method to allow end users to hook into Metamask's State Event Observer, and execute a callback when that changes. _(Eg: When an account is changed in the Metamask UI)_
  +
  +This method takes a callback as an argument, which will be added to the state events array. When the state changes, all the callbacks added to that array are called in order.
  +
  +When this is is called, it will receive a `state` Object as an only argument, Object which contains the new updated state.
  +
  +This utility method is useful to act on account changes from within a dApp. _(Eg: To logout a user)_
  +
  +**Usage examples:**
  +
  +Hook into the state change events with a simple callback:
  +```js
  +import { accountChangeHook } from '@colony/purser-metamask';
  +
  +const walletChangedCallback = ({ selectedAddress }) => {
  + console.log(`You changed your wallet. The new address is: ${selectedAddress}`);
  +};
  +
  +await accountChangeHook(walletChangedCallback);
  +```
  