docs: add a doc about the scroll strategies (#4630)

Adds a quick readme regarding the scroll strategies.

Author: crisbeto

src/lib/core/overlay/scroll/scroll-strategy.md

@@ -0,0 +1,27 @@
+# Scroll strategies
+
+## What is a scroll strategy?
+A scroll strategy is a class that describes how an overlay should behave if the user scrolls
+while the overlay is open. The strategy has a reference to the `OverlayRef`, allowing it to
+recalculate the position, close the overlay, block scrolling, etc.
+
+## Usage
+To associate an overlay with a scroll strategy, you have to pass in a `ScrollStrategy` instance
+to the `OverlayState`. By default, all overlays will use the `NoopScrollStrategy` which doesn't
+do anything:
+
+```ts
+let overlayState = new OverlayState();
+
+overlayState.scrollStrategy = new BlockScrollStrategy(this._viewportRuler);
+this._overlay.create(overlayState).attach(yourPortal);
+```
+
+## Creating a custom scroll strategy
+To set up a custom scroll strategy, you have to create a class that implements the `ScrollStrategy`
+interface. There are three stages of a scroll strategy's life cycle:
+
+1. When an overlay is created, it'll call the strategy's `attach` method with a reference to itself.
+2. When an overlay is attached to the DOM, it'll call the `enable` method on its scroll strategy,
+3. When an overlay is detached from the DOM or destroyed, it'll call the `disable` method on its
+scroll strategy, allowing it to clean up after itself.