Add technical documentation for RSNRU (#51435)

Summary:
Pull Request resolved: #51435

Changelog: [Internal]

Adding a technical documentation for the Runtime Shadow Node Referece Update (RSNRU) feature.

Reviewed By: rshest

Differential Revision: D74943809

fbshipit-source-id: cf5cc02adefa2635efb29e498747fc733711c2e4

Author: Nick Lefever

__docs__/README.md

@@ -45,6 +45,7 @@ TODO: Explain the different components of React Native at a high level.
     - UI / Fabric
       - Events
       - Shadow Tree Lifecycle
+        - [Runtime Shadow Node Reference Update](../packages/react-native/ReactCommon/react/renderer/core/__docs__/RSNRU.md)
       - Layout
       - Mounting
     - Native Modules / TurboModules