add contributing services docs

use-shopping-cart/CONTRIBUTING-SERVICES.md

@@ -0,0 +1,59 @@
+# Adding a service other than Stripe to use-shopping-cart
+
+In an effort to make this project ready for contributions that allow other payment services to be utilized with it, we created the `getCheckoutData` methods and the `checkoutHandler` function factory.
+
+This is a guide on how to add another payment service to use-shopping-cart.
+
+## Updating `getCheckoutData` to get your options for your service's API
+
+https://github.com/dayhaysoos/use-shopping-cart/blob/master/use-shopping-cart/src/util.js#L46
+
+The idea behind this abstraction is that you can make a function in `util.js`'s `getCheckoutData` object that will take in the `cart` and produce most if not all of the options that the service, such as Stripe, will need to go to or do a checkout. This function you create will need to match the name of your service as it is on the `cart` object, for `stripe` it'll be `stripe(cart) {/* your code */}`
+
+## Updating `checkoutHandler` to recognize your service's API
+
+Then, inside of `checkoutHandler` you'll need to modify it to recognize your provider. Right now we only have one, Stripe, so it looks like this:
+
+https://github.com/dayhaysoos/use-shopping-cart/blob/master/use-shopping-cart/src/util.js#L75
+
+When you write yours it might look like this:
+```js
+if (cart.stripe) serviceProperty = 'stripe'
+else if (cart.paypal) serviceProperty = 'paypal'
+```
+
+## My oops, necessary fix before it'll work:
+
+Then, as I forgot to do this in development myself, you'll need to change this line:
+https://github.com/dayhaysoos/use-shopping-cart/blob/master/use-shopping-cart/src/util.js#L97
+
+To say:
+```js
+if (needsCheckoutData) options = getCheckoutData[serviceProperty](cart)
+```
+
+## Updating a "checkout handler" with your service's API
+
+By this, I mean updating or creating a call to `checkoutHandler()` with the appropriate options and function for your service. In this instance, you'll likely just be updating `redirectToCheckout()` and `checkoutSingleItem()`. The format is as follows:
+
+https://github.com/dayhaysoos/use-shopping-cart/blob/master/use-shopping-cart/src/index.js#L136-L141
+
+It needs the `cart` as the first as the first argument, then we can get to the options.
+
+In the options, the `mode` property determines what modes should be allowed for use with this function, therefore, if a mode not in this list is used, an error will be thrown. Note: this might be better refactored to be specific to each service but we'll cross that bridge if we get there.
+
+Then below that, you can define a method/function that matches the `serviceProperty` name for your service, _`paypal` most likely._ The parameters for this function are `serviceObject, options, parameters`:
+
+- `serviceObject` - your service itself that you would use to do the checkout (`stripe.redirectToCheckout()`)
+- `options` - you created this via `getCheckoutData` and if you're in `checkout-session` mode you'll have a `sessionId` on it as well.
+- `parameters` - this is any extra options that you might need from the developer before you can go to the checkout, example below:
+
+https://github.com/dayhaysoos/use-shopping-cart/blob/master/use-shopping-cart/src/index.js#L143-L149
+
+### Other things to do:
+
+- Allow the developer to pass in the service object. It can be either the object itself or a promise resolving to that object.
+- Update TypeScript type definitions in the `.d.ts` file.
+- Update the documentation to explain how to use the added service with this library.
+
+And don't forget to make a Draft PR while you're working on all this so it's easier to track the progress 👍

use-shopping-cart/CONTRIBUTING.md

@@ -55,6 +55,10 @@ yarn test
 yarn dev:docs
 ```
 
-## Warning about editing the README
+### Warning about editing the README
 
-Please make all README edits to `/README.md` and when you're done copy them to `/use-shopping-cart/README.md`. This is done so that once the README updates are merged and published to NPM, you can see the README appear on both GitHub and NPM.
+Please make all README edits to `/README.md` and when you're done copy them to `/use-shopping-cart/README.md`. This is done so that once the README updates are merged and published to NPM, you can see the README appear on both GitHub and NPM.
+
+### Looking to add a payment service to use-shopping-cart
+
+If you're looking to add a new payment service like Square, Amazon Pay, PayPal, or even Shopify's Billing API, please [view this guide](https://github.com/dayhaysoos/use-shopping-cart/blob/master/use-shopping-cart/CONTRIBUTING-SERVICES.md) on how to do so.