[fix] Add troubleshooting section into the README (for issue #723)

README.md

@@ -120,3 +120,13 @@ A [development tutorial](doc/fr/development_tutorial-01.md) (in French) is also
 ## License
 
 This software is distributed under [GNU GPLv3](https://raw.github.com/duniter/cesium/master/LICENSE).
+
+## Troubleshooting
+
+#### I'm having errors on Ubuntu/Debian (desktop version)
+
+Install these dependencies:
+
+```
+sudo apt-get install -y libgconf-2-4
+```
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