chore: Update readme

Author: Ehesp

packages/shadcn/README.md

@@ -1 +1,13 @@
-TODO
+# Firebase UI for Web - Shadcn
+
+## Building the registry
+
+To build the registry, run the `build` script:
+
+```
+pnpm build
+```
+
+Note, the script run (`build.ts`) expects a domain, which replaces the `{{ DOMAIN }}` field within the 
+`registy-spec.json`. This enables building the registry for different domains without updating the domain
+in the actual `registry.json` file Shadcn expects.