Add an example of usage including a form

[Álvaro]

I believe it's not very clear what's the way of using it.
Adding an example with a form will help for it. In the current documentation only the server side is shown.

I found this example in stackoverflow which shows how to use it in the views as well:

app.use(require('body-parser')());
app.use(require('cookie-parser')('YOUR SECRET GOES HERE'));
app.use(require('express-session')());

app.use(require('csurf')());

app.get('/some-form', function(req, res){
    res.send('<form action="/process" method="POST">' +
        '<input type="hidden" name="_csrf" value="' + req.csrfToken() + '">' +
        'Favorite color: <input type="text" name="favoriteColor">' +
        '<button type="submit">Submit</button>' +
        '</form>');
});

app.post('/process', function(req, res){
    res.send('<p>Your favorite color is "' + req.body.favoriteColor + '".');
});

The use of csrfToken() is not explained in the docs.

[Douglas Wilson]

Duplicate of issue #7

Would you mind putting together a PR? Please make the HTML separate so it's readable instead of a string in JavaScript. You could make the layout similar to https://github.com/expressjs/method-override#examples