README: add documentation status indicator

Signed-off-by: Rouven Czerwinski <rouven@czerwinskis.de>

README.rst

@@ -1,7 +1,7 @@
 Welcome to labgridhelper
 ========================
 
-|license| |chat|
+|license| |docs-status| |chat|
 
 Purpose
 -------
@@ -13,6 +13,10 @@ service status information or the retrieval of ip addresses.
     :alt: LGPLv2.1
     :target: https://raw.githubusercontent.com/labgrid-project/labgrid/master/LICENSE
 
+.. |docs-status| image:: https://readthedocs.org/projects/labgridhelper/badge/?version=latest
+    :alt: documentation status
+    :target: https://labgridhelper.readthedocs.io/en/latest/?badge=latest
+
 .. |chat| image:: https://matrix.to/img/matrix-badge.svg
     :alt: chat
     :target: https://riot.im/app/#/room/#labgrid:matrix.org