Please correct some small errors in Readme

[dusmart]

In section Partial Decoding, there is one line which could not run correctly.

decoder := thrifter.NewDecoder(reader) should be decoder := thrifter.NewDecoder(reader, nil)

[dusmart]

When I tried to put some none-nil byte array to the second parameter, the program would always fail. Until I see your example in one file, I realize that the second parameter should be nil. Please update README.