Add some basic documentation for MVVM (#29316)

docs/MVVM.md

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+# MVVM
+
+General description of the pattern can be found [here](https://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93viewmodel). But the gist of it is that you divide your code into three sections:
+
+1. Model: This is where the business logic and data resides.
+2. View Model: This code exists to provide the logic necessary for the UI. It directly uses the Model code.
+3. View: This is the UI code itself and depends on the view model.
+
+If you do MVVM right, your view should be dumb i.e it gets data from the view model and merely displays it.
+
+### Practical guidelines for MVVM in element-web
+
+#### Model
+
+This is anywhere your data or business logic comes from. If your view model is accessing something simple exposed from `matrix-js-sdk`, then the sdk is your model. If you're using something more high level in element-web to get your data/logic (eg: `MemberListStore`), then that becomes your model.
+
+#### View Model
+
+1. View model is always a custom react hook named like `useFooViewModel()`.
+2. The return type of your view model (known as view state) must be defined as a typescript interface:
+    ```ts
+    inteface FooViewState {
+    	somethingUseful: string;
+    	somethingElse: BarType;
+    	update: () => Promise<void>
+    	...
+    }
+    ```
+3. Any react state that your UI needs must be in the view model.
+
+#### View
+
+1. Views are simple react components (eg: `FooView`).
+2. Views usually start by calling the view model hook, eg:
+    ```tsx
+    const FooView: React.FC<IProps> = (props: IProps) => {
+    	const vm = useFooViewModel();
+    	....
+    	return(
+    		<div>
+    			{vm.somethingUseful}
+    		</div>
+    	);
+    }
+    ```
+3. Views are also allowed to accept the view model as a prop, eg:
+    ```tsx
+    const FooView: React.FC<IProps> = ({ vm }: IProps) => {
+    	....
+    	return(
+    		<div>
+    			{vm.somethingUseful}
+    		</div>
+    	);
+    }
+    ```
+4. Multiple views can share the same view model if necessary.
+
+### Benefits
+
+1. MVVM forces a separation of concern i.e we will no longer have large react components that have a lot of state and rendering code mixed together. This improves code readability and makes it easier to introduce changes.
+2. Introduces the possibility of code reuse. You can reuse an old view model with a new view or vice versa.
+3. Adding to the point above, in future you could import element-web view models to your project and supply your own views thus creating something similar to the [hydrogen sdk](https://github.com/element-hq/hydrogen-web/blob/master/doc/SDK.md).
+
+### Example
+
+We started experimenting with MVVM in the redesigned memberlist, you can see the code [here](https://github.com/vector-im/element-web/blob/develop/src/components/views/rooms/MemberList/MemberListView.tsx).