DOC: fix deadlink, add doc badge

README.md

 # OTBTF: Orfeo ToolBox meets TensorFlow
 
 <p align="center">
-<img src="https://gitlab.irstea.fr/remi.cresson/otbtf/-/raw/develop/doc/images/logo.png" width="160px">
+<img src="images/logo.png" width="160px">
 <br>
 <a href="https://gitlab.irstea.fr/remi.cresson/otbtf/-/releases">
 <img src="https://gitlab.irstea.fr/remi.cresson/otbtf/-/badges/release.svg">
@@ -26,7 +26,7 @@ remote sensing imagery, easy to train in distributed environments.
 
 ## Documentation
 
-The documentation is available on [otbtf.readthedocs.io](otbtf.readthedocs.io).
+The documentation is available on [otbtf.readthedocs.io](https://otbtf.readthedocs.io).
 
 ## Use
 
@@ -52,4 +52,4 @@ You can also build OTBTF from sources (see the documentation)
   year={2018},
   publisher={IEEE}
 }
-```
\ No newline at end of file
+```

doc/index.md

@@ -9,6 +9,7 @@
 <a href="https://gitlab.irstea.fr/remi.cresson/otbtf/-/commits/master">
 <img src="https://gitlab.irstea.fr/remi.cresson/otbtf/badges/master/pipeline.svg">
 </a>
+<img src='https://readthedocs.org/projects/otbtf/badge/?version=latest' alt='Documentation Status' />
 <a href="LICENSE">
 <img src="https://img.shields.io/badge/License-Apache%202.0-blue.svg">
 </a>