add tutorial and cname option to README

I finally got a solution of my issue. https://github.com/pajtai/grunt-build-gh-pages/issues/15. I modified document more friendly for newbies like me. 

However, I got confused some content of your document. Please give some example or clarify its meaning

1. what's difference with options and your_target? can you give me example?
	```
	  options: {
	      // Task-specific options go here.
	    },
	    your_target: {
	      // Target-specific file lists and/or options go here.
	    },
	```

2. What's the meaning of following sentence?
	
	> In this example, the default options are used to do something with whatever. So if the testing file has the content Testing and the 123 file had the content 1 2 3, the generated result would be Testing, 1 2 3.