DOC: move contribute section in index

doc/index.md

-# Pyotb: Orfeo Toolbox for Python
+# pyotb: Orfeo Toolbox for Python
 
 pyotb is a Python extension of Orfeo Toolbox. It has been built on top of the 
 existing Python API of OTB, in order 
@@ -9,7 +9,7 @@ to make OTB more Python friendly.
 ## Get started
 
 - [Installation](installation.md)
-- [How to use pyotb](quickstart.md)
+- [Quick start](quickstart.md)
 - [Useful features](features.md)
 - [Functions](functions.md)
 - [Interaction with Python libraries (numpy, rasterio, tensorflow)](interaction.md)
@@ -27,8 +27,17 @@ to make OTB more Python friendly.
 - [Managing loggers](managing_loggers.md)
 - [Troubleshooting & limitations](troubleshooting.md)
 
-
 ## API
 
 - See the API reference. If you have any doubts or questions, feel free to ask
-on github or gitlab!
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+on github or gitlab!
+
+## Contribute
+
+Contributions are welcome !
+Open a PR/MR, or file an issue if you spot a bug or have any suggestion:
+
+- [Github](https://github.com/orfeotoolbox/pyotb) 
+- [Orfeo ToolBox GitLab instance](https://gitlab.orfeo-toolbox.org/nicolasnn/pyotb).
+
+Thank you!
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