Add a new customer to Stripe when the user signs up.
### How this function works
This function will run every time a new user signs up in Firebase. It will create a new Stripe customer associated with the new user.
### Requirements
* You must have previous knowledge of **Firebase**.
* You must have previous knowledge of **Firestore**.
* You must have previous knowledge of **Stripe**.
### Function details
When a new user is created in Firebase, this function will create a new Stripe customer, and store its info in the collection referenced by the path provided. It will use the Firebase user ID as the id of the stored customer document. For example, for the Firebase user with uid, the function will store its associated customer in `STRIPE_CUSTOMERS_COLLECTION_PATH/{uid}`.
To install this function you need to enter the values of the following environment variables:
* **STRIPE_SECRET_KEY**: Your Stripe secret API key. See [https://stripe.com/docs/keys](https://stripe.com/docs/keys).
* **STRIPE_CUSTOMERS_COLLECTION_PATH**: The collection path where the Stripe customers are stored.
### Resources
- [Stripe customers](https://stripe.com/docs/api/customers)