add some documentation for streams

See also questions and discussions on this topic:
http://stackoverflow.com/a/17536438/859591
https://dev.twitter.com/discussions/8081
https://dev.twitter.com/discussions/8110
Merge pull request #167 from lumbric/master

Add stream documenation
It was very difficult to find information on this topic. Now that I figured out how to get direct messages, I added it to the README.

See also questions and discussions on this topic:
http://stackoverflow.com/a/17536438/859591
https://dev.twitter.com/discussions/8081
https://dev.twitter.com/discussions/8110