Explain that uploads are persisted

src/vmod_tus.vcc

@@ -49,6 +49,21 @@ request body to be sent to a backend.
 
 By this VCL integration, http behaviour remains highly customizable.
 
+Persistent Storage
+------------------
+
+Uploads using this module are persistent across Varnish and system
+restarts, given that the ``basedir`` argument to `tus.server()`_
+specifies a directory which remains intact across Varnish and system
+restarts, respectively.
+
+When the first `tus.server()`_ is initialized, all files in
+``basedir`` which match the vmod_tus naming convention (prefixed by
+``tus_``) are checked and reused if they appear consistent and have
+not yet expired.
+
+Concat final uploads are not restored, but parts are.
+
 CORS
 ----