Add a user to Firestore when the user signs up.
### How this function works
This function will allow you to store the user data in a Firestore collection when a new user signs up.
### Requirements
* You must have previous knowledge of **Firestore**.
* You need a Firebase project with **Firestore** available.
### Function details
This function listens when a new user signs up and then adds this user record in the specific collection.
To install this function you need to enter the values of the following environment variables:
* **FIRESTORE_COLLECTION_PATH**: The collection path that will be used to save the users.