Unnecessary use of cat in README

The following (taken from the README) encourages using cat when redirections would be more appropriate and (marginally) faster:
```shell
$ cat example.docopt | python docopt_c.py > docopt.c
```

A functionally equivalent, but slightly faster and cleaner version would be:

```shell
$ python docopt_c.py > docopt.c < example.docopt
```