[SDK-3784] docs: add migration guide

UPDATING.md

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+# Upgrade / Migration Guide
+
+## Version 2.x to 3.x
+
+### Replacing `configureAbly` with `AblyProvider` 
+
+In versions 1 and 2 of our react-hooks, we exported a function called `configureAbly` which was used to register an Ably client instance to global state.
+This caused a few issues (most notably it made the hooks difficult to use with hot module reloading), so we have replaced the global configuration function with a context provider (`AblyProvider`)
+The simplest way to use the context provider is to create your own ably-js client outside and then pass it as a prop to the `AblyProvider`.
+All child components of the `AblyProvider` will then be able to use the hooks, making use of the provided Ably client instance. For this reason, we recommend putting the `AblyProvider` high up in your component tree, surrounding all components which may need to use Ably hooks.
+
+For example, replace this:
+```jsx
+configureAbly(options);
+```
+
+With this:
+```jsx
+const client = new Ably.Realtime.Promise(options);
+
+return <AblyProvider client={ably}>
+  {children}
+</AblyProvider>
+```
+
+You may also provide the client options directly to the `AblyProvider` so that the client is created automatically. If you use this prop the client will be automatically closed when the `AblyProvider` is unmounted.
+
+```jsx
+return <AblyProvider options={options}>
+  {children}
+</AblyProvider>
+```
+
+If you were already using multiple Ably clients in the same react application, you may pass in an optional `id` prop to the provider, which you can then pass to the hooks to specify which Ably client instance the hook should use:
+```jsx
+const client = new Ably.Realtime.Promise(options);
+
+return <AblyProvider client={ably} id="foo">
+  {children}
+</AblyProvider>
+
+// in a child component:
+useChannel({channelName: 'my_channel', id: 'foo'}, (msg) => {
+  console.log(msg);
+});
+```