docs: link to the internals doc from CONTRIBUTING.md

Make the architecture overview discoverable from the main contributor entry point.

Signed-off-by: Kévin Dunglas <kevin@dunglas.fr>

Author: dunglas

CONTRIBUTING.md

@@ -1,5 +1,7 @@
 # Contributing
 
+For an overview of FrankenPHP's architecture (thread types, state machine, CGO boundary, request flow), see the [Internals documentation](docs/internals.md).
+
 ## Compiling PHP
 
 ### With Docker (Linux)