Add Stripe Keys to Config

Back in the [Setup a Stripe account]({% link _chapters/setup-a-stripe-account.md %}) chapter, we had two keys in the Stripe console. The Secret key that we used in the backend and the Publishable key. The Publishable key is meant to be used in the frontend.

Add the following line to the `export` block of `src/config.js`.
STRIPE_KEY: "YOUR_STRIPE_PUBLIC_KEY",

Make sure to replace, YOUR_STRIPE_PUBLIC_KEY with the Publishable key from the [Setup a Stripe account]({% link _chapters/setup-a-stripe-account.md %}) chapter.

Next, we'll build our billing form.