[Documentation] Offsite Payments / Universal Links (#143)

* Improve documentation

* Update code example for universal links

* Add section for diagnostics

Author: markmur

CONTRIBUTING.md

@@ -1,5 +1,10 @@
 # Contributing
 
+We welcome code contributions, feature requests, and reporting of issues. Please
+see [guidelines and instructions](.github/CONTRIBUTING.md).
+
+---
+
 This repo is subdivided into 3 parts using yarn workspaces:
 
 - The base repo (workspace name = `checkout-sheet-kit-react-native`)
@@ -94,3 +99,44 @@ yarn sample test:ios
 # Run Java tests for the Native Module
 yarn sample test:android
 ```
+
+## Running the sample app
+
+To run the sample app in this repo, first clone the repo and run the following
+commands at the root of the project directory.
+
+### Install NPM dependencies
+
+```sh
+yarn
+```
+
+### Install Cocoapods
+
+```sh
+yarn pod-install sample/ios
+```
+
+### Build the local module
+
+```sh
+yarn module build
+```
+
+### Update the dotenv file
+
+Replace the details in the `sample/.env.example` file and rename it to
+`sample/.env`
+
+```
+# Storefront Details
+STOREFRONT_DOMAIN="YOUR_STORE.myshopify.com"
+STOREFRONT_ACCESS_TOKEN="YOUR_PUBLIC_STOREFRONT_ACCESS_TOKEN"
+STOREFRONT_VERSION="2024-04"
+```
+
+### Start the sample app
+
+```sh
+yarn sample start
+```