move old docs over

site/content/docs/13-base-urls.md

@@ -0,0 +1,28 @@
+---
+title: Base URLs
+---
+
+Ordinarily, the root of your Sapper app is served at `/`. But in some cases, your app may need to be served from a different base path — for example, if Sapper only controls part of your domain, or if you have multiple Sapper apps living side-by-side.
+
+This can be done like so:
+
+```js
+// app/server.js
+
+express() // or Polka, or a similar framework
+  .use(
+    '/my-base-path', // <!-- add this line
+    compression({ threshold: 0 }),
+    serve('assets'),
+    sapper.middleware()
+  )
+  .listen(process.env.PORT);
+```
+
+Sapper will detect the base path and configure both the server-side and client-side routers accordingly.
+
+If you're [exporting](guide#exporting) your app, you will need to tell the exporter where to begin crawling:
+
+```bash
+sapper export --basepath my-base-path
+```