README: Add rtd and codecov badges

[ci skip]

README.rst

@@ -2,6 +2,10 @@
 scikit-ci-addons
 ===============================
 
+.. image:: https://readthedocs.org/projects/scikit-ci-addons/badge/?version=latest
+    :target: http://scikit-ci-addons.readthedocs.io/en/latest/?badge=latest
+    :alt: Documentation Status
+
 .. image:: https://ci.appveyor.com/api/projects/status/gr60jc9hkjlqoo4a?svg=true
     :target: https://ci.appveyor.com/project/scikit-build/scikit-ci-addons/branch/master
 
@@ -11,6 +15,9 @@ scikit-ci-addons
 .. image:: https://img.shields.io/travis/scikit-build/scikit-ci-addons.svg?maxAge=2592000
     :target: https://travis-ci.org/scikit-build/scikit-ci-addons
 
+.. image:: https://codecov.io/gh/scikit-build/scikit-ci-addons/branch/master/graph/badge.svg
+  :target: https://codecov.io/gh/scikit-build/scikit-ci-addons
+
 scikit-ci-addons is a command line tool and a set of scripts useful to install
 prerequisites for building Python extension on CI services.