minor readme update.

Took me a minute to realize i needed to quote regex params so I added it in readme.

```
$ cat test.js
var pupil = require('pupil'),
    rules = {bar: "required && regex([0-9]{5})"},
    doc = {bar: "0000"};

console.log(pupil.validate(rules, doc));

$ node test.js 

/tmp/node_modules/pupil/src/parser.js:101
                if ( ! accept.identifier) { throw new ParserException("Unexpec
                                                  ^
[object Object]
```