Improve documentation for Stripe payments provider

[aniav]

Currently documentation for Stripe payment backand states that developers should attach form.media to the payment form. But this setup is insufficient and won't ever produce a successfull payment. In saleor stripe payment form template includes stripe payment button that seems to be necessary and should be included in the documentation

    <script
        src="https://checkout.stripe.com/checkout.js" class="stripe-button"
        data-key="{{ form.stripeToken.field.widget.attrs.data_key }}"
        data-image="{{ STATIC_URL }}img/logo.png"
        data-name="Site Name"
        data-description="{{ form.stripeToken.field.widget.attrs.data_description }}"
        data-amount="{{ form.stripeToken.field.widget.attrs.data_amount }}"
        data-currency="{{ form.stripeToken.field.widget.attrs.data_currency }}">
    </script>

[patrys]

Please check 0.6.0