feature: support .spa files for Single Page Apps

README.md

@@ -69,6 +69,19 @@ listening.
  - `SERVERUS_PUT_PATHS_ALLOWED`: a list of ;-separated directories for which file uploads via PUT are allowed
  - `SERVERUS_DELETE_PATHS_ALLOWED`: a list of ;-separated directories for which file deletions via DELETE are allowed
 
+### Singe-Page Applications (SPA)
+
+If you place a `.spa` file under the root, that directory will try to return an `index.html` or `index.htm` file that lives in it for any requests that are relative to it. For example:
+
+```
+  www/
+    app/
+      .spa
+      index.html
+```
+
+If you have this file structure (assuming `www` is the root), a `GET` to `/app/foo/bar` will return the `index.html` file in the `app/` directory. (Which would then presumably handle the url in `window.location` appropriately.)
+
 ### Typescript Handling
 
 These types and interface define the default serverus Typescript handler's expected