Update README.md

Added .run() because getJoins() returns a query object not a promise to chain. Otherwise below error is returned 
```
Error: The method `then` is not defined on Query. Did you forgot `.run()` or `.execute()`?
    at Query.then (/Users/chan/Development/Workspaces/Node-Workspace/sequel-io/node_modules/thinky/lib/query.js:291:11)
```