add docs badge

Author: mr-c

README.rst

@@ -2,7 +2,7 @@
 Common Workflow Language tool description reference implementation
 ==================================================================
 
-|Linux Status| |Coverage Status|
+|Linux Status| |Coverage Status| |Docs Status|
 
 PyPI: |PyPI Version| |PyPI Downloads Month| |Total PyPI Downloads|
 
@@ -25,13 +25,13 @@ Quay.io (Docker): |Quay.io Container|
    :target: https://codecov.io/gh/common-workflow-language/cwltool
 
 .. |PyPI Version| image:: https://badge.fury.io/py/cwltool.svg
-    :target: https://badge.fury.io/py/cwltool
+   :target: https://badge.fury.io/py/cwltool
 
 .. |PyPI Downloads Month| image:: https://pepy.tech/badge/cwltool/month
    :target: https://pepy.tech/project/cwltool
 
 .. |Total PyPI Downloads| image:: https://static.pepy.tech/personalized-badge/cwltool?period=total&units=international_system&left_color=black&right_color=orange&left_text=Total%20PyPI%20Downloads
- :target: https://pepy.tech/project/cwltool
+   :target: https://pepy.tech/project/cwltool
 
 .. |Conda Version| image:: https://anaconda.org/conda-forge/cwltool/badges/version.svg 
    :target: https://anaconda.org/conda-forge/cwltool
@@ -42,6 +42,10 @@ Quay.io (Docker): |Quay.io Container|
 .. |Quay.io Container| image:: https://quay.io/repository/commonwl/cwltool/status
    :target: https://quay.io/repository/commonwl/cwltool
 
+.. |Docs Status| image:: https://readthedocs.org/projects/cwltool/badge/?version=latest
+   :target: https://cwltool.readthedocs.io/en/latest/?badge=latest
+   :alt: Documentation Status
+
 This is the reference implementation of the Common Workflow Language.  It is
 intended to be feature complete and provide comprehensive validation of CWL
 files as well as provide other tools related to working with CWL.