add usage based billing sample

usage-based-subscriptions/README.md

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+# Subscriptions with metered usage
+
+Create a subscription for an online service with metered usage options, and work with Stripe Elements to host a payment form on your servers.
+This sample shows how to create a customer, set up a card for recurring use, and subscribe them to a subscription plan with
+[Stripe Billing](https://stripe.com/billing).
+
+
+**Demo**
+
+See a hosted version of the [sample](https://2ow1q.sse.codesandbox.io/) in test mode or [fork on codesandbox.io](https://codesandbox.io/s/great-wood-2ow1q)
+
+The hosted demo is running in test mode -- use `4242424242424242` as a test card number with any CVC + future expiration date.
+
+Use the `4000000000003220` test card number to trigger a 3D Secure challenge flow.
+
+Read more about test cards on Stripe at https://stripe.com/docs/testing.
+
+<img src="subscription-with-metered-usage.png" alt="Preview of recipe" style="max-width:25%;">
+
+### Features:
+
+- 💳Securely collect card details
+- 🔒Save the payment method details to a customer
+- 🚫Handle payment failures
+- 💰Subscribe the customer to a subscription plan
+- ➕Upgrade and downgrade on plans
+
+## How to run locally
+
+This sample includes [5 server implementations](server/) in our most popular languages. Follow the steps below to run one of the servers locally.
+
+**1. Clone and configure the sample**
+
+The Stripe CLI is the fastest way to clone and configure a sample to run locally.
+
+**Using the Stripe CLI**
+
+If you haven't already installed the CLI, follow the [installation steps](https://github.com/stripe/stripe-cli#installation) in the project README. The CLI is useful for cloning samples and locally testing webhooks and Stripe integrations.
+
+In your terminal shell, run the Stripe CLI command to clone the sample:
+
+```
+stripe samples create subscriptions-with-fixed-price
+```
+
+The CLI will walk you through picking your integration type, server and client languages, and configuring your `.env` config file with your Stripe API keys.
+
+**Installing and cloning manually**
+
+If you do not want to use the Stripe CLI, you can manually clone and configure the sample:
+
+```
+git clone [email protected]:stripe-samples/subscription-with-fixed-price.git
+```
+
+Copy the `.env.example` file into a file named `.env` in the folder of the server you want to use. For example:
+
+```
+cp .env.example server/node/.env
+```
+
+You will need a Stripe account in order to run the demo. Once you set up your account, go to the Stripe [developer dashboard](https://stripe.com/docs/development#api-keys) to find your API keys.
+
+```
+STRIPE_PUBLISHABLE_KEY=<replace-with-your-publishable-key>
+STRIPE_SECRET_KEY=<replace-with-your-secret-key>
+```
+
+`STATIC_DIR` tells the server where the client files are located and does not need to be modified unless you move the server files.
+
+**2. Create Products and Plans on Stripe**
+
+This sample requires a [Plan](https://stripe.com/docs/api/plans/object) ID to create the subscription. Products and Plans are objects on Stripe that you use to model a subscription.
+
+You can create Products and Plans [in the Dashboard](https://dashboard.stripe.com/products) or with the [API](https://stripe.com/docs/api/plans/create). Create a Plan to run this sample and add it to your `.env`.
+
+**3. Follow the server instructions on how to run:**
+
+Pick the server language you want and follow the instructions in the server folder README on how to run.
+
+```
+cd server/node # there's a README in this folder with instructions
+npm install
+npm start
+```
+
+**4. [Optional] Run a webhook locally:**
+
+You can use the Stripe CLI to forward webhook events to your server running locally.
+
+If you haven't already, [install the CLI](https://stripe.com/docs/stripe-cli) and [link your Stripe account](https://stripe.com/docs/stripe-cli#link-account).
+
+```
+stripe listen --forward-to localhost:4242/webhook
+```
+
+The CLI will print a webhook secret key to the console. Set `STRIPE_WEBHOOK_SECRET` to this value in your .env file.
+
+You should see events logged in the console where the CLI is running.
+
+When you are ready to create a live webhook endpoint, follow our guide in the docs on [configuring a webhook endpoint in the dashboard](https://stripe.com/docs/webhooks/setup#configure-webhook-settings).
+
+## FAQ
+
+Q: Why did you pick these frameworks?
+
+A: We chose the most minimal framework to convey the key Stripe calls and concepts you need to understand. These demos are meant as an educational tool that helps you roadmap how to integrate Stripe within your own system independent of the framework.
+
+Q: Can you show me how to build X?
+
+A: We are always looking for new recipe ideas, please email [email protected] with your suggestion!
+
+## Author(s)
+
+- [@ctrudeau-stripe](https://twitter.com/trudeaucj)