Add doc about sandbox mode

README.md

@@ -10,7 +10,7 @@ This SDK is **not** for a server-side NodeJS app.
 
 ## Installation
 
-Add the following `script` tags to all pages where you need the SDK to be available:
+Add the following `script` tags to all pages where you need to call the Pi Apps SDK:
 
 ```html
 <script src="https://sdk.minepi.com/pi-sdk.js"></script>

SDK_reference.md

@@ -1,5 +1,37 @@
 # Client SDK reference:
 
+## Initialization
+
+Add the following `script` tags to all pages where you need to call the Pi Apps SDK:
+
+```html
+<script src="https://sdk.minepi.com/pi-sdk.js"></script>
+<script>Pi.init({ version: "2.0" })</script>
+```
+
+The config object passed to the init function accepts the following keys:
+* `version` (string, required) - this is required to ensure compatibility of your app with newer SDK versions that might bring
+  breaking changes (in which case breaking changes will be implemented under a higher version number)
+* `sandbox`: (boolean, optional) - this enables you to configure the SDK to run in the sandbox.
+
+### Using the SDK in sandbox mode:
+
+```html
+<script src="https://sdk.minepi.com/pi-sdk.js"></script>
+<script>Pi.init({ version: "2.0", sandbox: true })</script>
+```
+
+You may now run your app in the sandbox environment (https://sandbox.minepi.com), provided you've configured
+a development URL in the developer portal. Visit the developer portal by opening develop.pi in the Pi Browser
+to configure this and view your Sandbox URL.
+
+> Typically, if you're using a framework or a boilerplate that supports it, you should be able to set up your
+> sandbox flag to match your development environment. For example, most good Node boilerplates will set up the
+> value of `process.env.NODE_ENV` to either `"development"` or `"production"`, and you could do something like:
+> `Pi.init({ version: "2.0", sandbox: <%= process.env.NODE_ENV !== 'production' %> })`. This depends on your
+> setup, but running the Pi SDK in sandbox mode will generally happen whenever your app is running in development.
+
+
 ## Authentication
 
 ```typescript