Add mention in docs

Author: leplatrem

docs/fastapi.rst

@@ -44,7 +44,7 @@ To install ``python-dockerflow``'s FastAPI support please follow these steps:
      app = FastAPI()
      app.include_router(router)
      app.add_middleware(MozlogRequestSummaryLogger)
-     app.add_middleware(CorrelationIdMiddleware)
+     app.add_middleware(CorrelationIdMiddleware)  # see snok/asgi-correlation-id
 
 #. Make sure the app root path is set correctly as this will be used
    to locate the ``version.json`` file that is generated by
@@ -308,6 +308,9 @@ for at least the ``request.summary`` logger:
         }
     })
 
+
+A unique ID is added to each log message, using the `asgi-correlation-id <https://github.com/snok/asgi-correlation-id>`_ package.
+
 .. _fastapi-static:
 
 Static content