docs(readme): add readthedocs.org badge into project readme

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: I9d65acdb1fd814b5d65c0592838aec181c603ee3
diff --git a/docs/readme.rst b/docs/readme.rst
index c455de6..095faac 100644
--- a/docs/readme.rst
+++ b/docs/readme.rst
@@ -4,6 +4,10 @@
 .. image:: https://scan.coverity.com/projects/28411/badge.svg
    :target: https://scan.coverity.com/projects/tf-rmm-tf-rmm
 
+.. image:: https://readthedocs.org/projects/tf-rmm/badge/?version=latest
+    :target: https://tf-rmm.readthedocs.io/en/latest/?badge=latest
+        :alt: Documentation Status
+
 #################
 Readme for TF-RMM
 #################