docs: add top-level package diagram

Author: gadomski

README.md

@@ -1,2 +1,13 @@
 # pystapi
+
 Monorepo for Python Satellite Tasking API (STAPI) Specification packages
+
+## Packages
+
+This diagram is aspirational while we build out things during the 2025 STAPI sprint in Lisbon.
+
+```mermaid
+graph
+    stapi-pydantic --> pystapi-client --> pystapi-validator
+    stapi-pydantic --> stapi-fastapi
+```