Update README

Added example on how to search for videos based on query and user.

The desired query:
    https://www.youtube.com/user/Vsauce/search?query=some+surprising+things

Searching for videos based on query and user doesn't work with the following example:
```client.videos_by(:user => 'vsauce', :query => "some surprising things")```

After looking playing around with different parameters I found the solution. 
```client.videos_by(:author => 'vsauce', :query => "some surprising things")```

Not sure why this doesn't work with user but author, but I added an example in README for future users stumbling across this problem.

https://github.com/kylejginavan/youtube_it/issues/223