compilation.md

@@ -20,6 +20,12 @@ cd build
 cmake ..
 make
 ```
+On Ubuntu 18 you may need to run :
+```shell
+cd build
+cmake .. -DPYTHON_EXECUTABLE=/usr/bin/python3
+make
+```
 
 Instead of simply running `cmake`, you can run `ccmake` or `cmake-gui` to help you changing some
 configuration parameters. For examples people interested only in rigid computation can change