Update the README

- Describe the `Docker::Container#top` method
- Change the compatible version of Docker to `0.5.1`
- Change the documentation link to version `0.1.4`
Minor version bump (1.4.0)

Although there were no backward incompatible changes since `1.3.1`,
Docker has changed the output of its API in the most recent version.
Also, the `Docker::Container#top` method was added.