update SUMMARY.md with new stories

Author: nikomatsakis

src/SUMMARY.md

@@ -24,6 +24,7 @@
    - [😱 Status quo](./vision/status_quo.md)
       - [Template](./vision/status_quo/template.md)
       - [Alan runs into stack trouble](./vision/status_quo/alan_runs_into_stack_trouble.md)
+      - [Barbara makes their first foray into async](./vision/status_quo/barbara_makes_their_first_steps_into_async.md)
       - [Grace deploys her service and hits obstacles](./vision/status_quo/grace_deploys_her_service.md)
    - [✨ Shiny future](./vision/shiny_future.md)
       - [Template](./vision/shiny_future/template.md)