Add plug-ins section and link to swift-html (#164)

Author: stephencelis

README.md

@@ -176,6 +176,12 @@ dependencies: [
   - **SceneKit, SpriteKit, and WebKit support.** Most snapshot testing libraries don't support these view subclasses.
   - **`Codable` support**. Snapshot encodable data structures into their [JSON](Documentation/Available-Snapshot-Strategies.md#json) and [property list](Documentation/Available-Snapshot-Strategies.md#plist) representations.
   - **Custom diff tool integration**.
+
+## Plug-ins
+
+  - [swift-html](https://github.com/pointfreeco/swift-html) is a Swift DSL for type-safe, extensible, and transformable HTML documents and includes an `HtmlSnapshotTesting` module to snapshot test its HTML documents.
+
+Have you written your own SnapshotTesting plug-in? [Add it here](https://github.com/pointfreeco/swift-snapshot-testing/edit/master/README.md) and submit a pull request!
 
 ## Related Tools