README: format warning message

Author: ntarocco

README.rst

@@ -10,11 +10,10 @@
  Invenio Framework v3
 ======================
 
-Warning
--------
-The main development effort is currently on the `InvenioRDM project <https://inveniosoftware.org/products/rdm/>`_
-and there will be no new releases of Invenio framework. However, each Invenio module is
-actively maintained as part of `InvenioRDM. <https://github.com/inveniosoftware/invenio-app-rdm>`_
+.. warning::
+    The main development effort is currently on the `InvenioRDM project <https://inveniosoftware.org/products/rdm/>`_
+    and there will be no new releases of Invenio framework. However, each Invenio module is
+    actively maintained as part of `InvenioRDM. <https://github.com/inveniosoftware/invenio-app-rdm>`_
 
 **Open Source framework for large-scale digital repositories.**