Add examples of flag use to README

It took me forever to figure out how to use the flag functionality; even in the generate godoc for glog, the instruction to call flag.Parse() is very easy to overlook. 

By putting the information in the pull request in the README it will help prevent confusion to people either new to golang or who lack knowledge of the flag package. 
You must call flag.Parse in your program's main function for glog's command-line flags to take effect.

I think the rest of this comment is overkill.