Tools: add readme for completion

Tools/completion/completion.md

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+## on Bash
+To install the completion, you should either, source the main script on your current terminal or source the main script on your bash configuration.
+
+### Direct use
+From ArduPilot root directory, simply use :
+```` bash
+source ./Tools/completion/completion.bash
+````
+And now completion works from your terminal instance. If you close the terminal, the completion feature is removed.
+
+### Permanent use
+Edit you `.bashrc` file, it is on your Home directory but it is a hidden file (CTRL+H on Ubuntu to reveal them). Then put at the end of the file :
+```` bash
+source PATH_TO_ARDUPILOT_DIRECTORY/Tools/completion/completion.bash
+````
+where PATH_TO_ARDUPILOT_DIRECTORY is the path to ArduPilot directory.
+
+### Usage
+You can now abuse of your TAB key on `waf` and `sim_vehicle.py` call ! See the video at the end.
+
+## On ZSH
+Zsh don't allow live loading of completion. So you have to source the completion script in your `.zshrc` file. Like for Bash, you will find it hiding on your home !
+Put at the end of the file :
+```` bash
+source PATH_TO_ARDUPILOT_DIRECTORY/Tools/completion/completion.zsh
+````
+where PATH_TO_ARDUPILOT_DIRECTORY is the path to ArduPilot directory. Notice the difference, the extension is `.zsh` !