example: add README to Svelte example (#4011)

examples/svelte-example/README.md

@@ -1,3 +1,16 @@
-# `svelte-example`
+# Uppy with Svelte
 
-More info coming soon...
+## Run it
+
+To run this example, make sure you've correctly installed the **repository root**:
+
+```sh
+corepack yarn install
+corepack yarn build
+```
+
+Then, again in the **repository root**, start this example by doing:
+
+```sh
+corepack yarn workspace @uppy-example/svelte-app dev
+```