docs(release): document REACT_LOGIN + API_URL_PREFIX in the README config block (v1 polish)

The two settings that landed this release window aren't visible in the
README's optional-configuration snippet — add them with one-line
inline docs so consumers landing on the PyPI page see the full v1
surface at a glance:

- `REACT_LOGIN` — True by default (post-#561); set False to opt back
  into the legacy admin HTML login.
- `API_URL_PREFIX` — None by default; point the SPA at a separately-
  mounted django-admin-rest-api when the consumer doesn't want the
  inline include (#559).

Author: martin-castro-laminr-ai

README.md

@@ -141,6 +141,16 @@ DJANGO_ADMIN_REACT = {
                                 # active states. Hex only (validated);
                                 # injected as the --dar-primary CSS var, so
                                 # rebranding needs no React rebuild.
+
+    # Auth + API mount
+    "REACT_LOGIN": True,        # bool — React-rendered login is the default;
+                                # the SPA shell is served to anonymous users
+                                # and posts to /api/v1/login/. Set False to
+                                # opt back into the legacy admin HTML login.
+    "API_URL_PREFIX": None,     # str | None — point the SPA at a separately-
+                                # mounted django-admin-rest-api (e.g.
+                                # "/api/api/v1/"). Default None keeps the
+                                # inline include the package ships today.
 }
 ```