README improvements

[lutovich]

PR adds following entries to the README.md:

• need for driver.close()
• ordering of onNext, onCompleted and onError callbacks
• existence of driver.onCompleted and driver.onError callbacks

Closes #151 #185 #120

[zhenlineo]

LGTM, I suggest also include driver.close in the example to be more clear about it.

[lutovich]

@zhenlineo added driver.close() to the "Usage examples" section.