README.txt: Mention put_url restriction

The PUT URL domain is used as part of the mod_http_upload process name.
This way, the responsible process can be queried during the HTTP PUT
request.  As a result, a given PUT URL domain cannot be used by multiple
virtual hosts.

mod_http_upload/README.txt

@@ -92,6 +92,8 @@ The configurable mod_http_upload options are:
   This option specifies the initial part of the PUT URLs used for file
   uploads.  The keyword @HOST@ is replaced with the virtual host name.
 
+  NOTE: Different virtual hosts cannot use the same PUT URL domain.
+
 - get_url (default: $put_url)
 
   This option specifies the initial part of the GET URLs used for