Add LDK architecture to references

Author: jkczyz

docs/references.md

@@ -18,3 +18,8 @@ a lightning node using LDK.
 
 These docs are mainly geared towards how Swift could call LDK C bindings directly, but still may
 provide a useful overview of Rust Lightning in the context of language bindings.
+
+### [LDK Architecture](https://docs.google.com/drawings/d/1Ql-q5gyrPnJhi7z_D39jayG0HEEVh6UEY1eULXb03Eg/edit?usp=sharing)
+
+Gives a high-level organization of LDK and how the pieces fit together. Variations of this diagram
+are used throughout the site. This is the primary source and is still a work in progress.