Update README.markdown

The docs about `queue.freeze` and `queue.defrost` were a bit misleading (at least for me). It said the methods are not asynchronous, so I was trying to call `freeze` and exit process immediately. In effect I got an empty .json file. This changes aim at clearing this.
Merge pull request #87 from lzrski/patch-1

Update README.markdown section about freeze and defrost