[saas-microservices] Add mermaid diagram to show route splitting (#1191)

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### Type of Change

- [ ] New Example
- [ ] Example updates (Bug fixes, new features, etc.)
- [ ] Other (changes to the codebase, but not to examples)

### New Example Checklist

- [ ] 🛫 `npm run new-example` was used to create the example
- [ ] 📚 The template wasn't used but I carefuly read the [Adding a new
example](https://github.com/vercel/examples#adding-a-new-example) steps
and implemented them in the example
- [ ] 📱 Is it responsive? Are mobile and tablets considered?

Author: mknichel

solutions/saas-microservices/README.md

@@ -23,9 +23,14 @@ There are 3 separate applications in this example:
 
 These all run under the same domain. Paths to each application are routed using Vercel's [microfrontends](https://vercel.com/docs/microfrontends) support:
 
-- `/api/dashboard/*` - Routes to the `api-dashboard` backend
-- `/api/users/*` - Routes to the `api-users` backend
-- Everything else - Routes to the `dashboard` application
+```mermaid
+graph TD
+    Request --> Vercel{Vercel}
+
+    Vercel -->|"/api/dashboard/*"| ApiDashboard[api-dashboard]
+    Vercel -->|"/api/users/*"| ApiUsers[api-users]
+    Vercel -->|"Everything else"| DashboardApp[Dashboard]
+```
 
 ## Running Locally