Update chapter6/6-1-static-file-server.js

"The problem with fs.readFile, though, is that it wants to read the file completely into memory before making it available.Documents served over the Web can be quite large. In addition, there can be many requests for a document at any given time. Something like fs.readFile just won’t scale."
上面是一段摘自"learning node"中的一段，其实说的就是readfile的缺点
对于readfile的话，我们必须首先将整个文件都读入到内存然后才能进行使用，这就有一个问题，有的时候文件会非常大，并且有的时候会有不止一个请求发过来，这个时候readfile就不太好用了。
fs.CreateReadStream的好处：
1:it’s a simple matter to just pipe the file contents directly to the HTTP response object.
2:The read stream has two events of interest: open and error. 
3: Since the stream sends an end signal when it’s finished, we don’t need to use the end method call with the read stream: