Add command completion docs (#410) (#597)

Co-authored-by: Rory Flynn <[email protected]>

docs/command_completion.md

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+# Command Completion 
+
+MVT utilizes the [Click](https://click.palletsprojects.com/en/stable/) library for creating its command line interface. 
+
+Click provides tab completion support for Bash (version 4.4 and up), Zsh, and Fish.
+
+To enable it, you need to manually register a special function with your shell, which varies depending on the shell you are using.
+
+The following describes how to generate the command completion scripts and add them to your shell configuration. 
+
+`You will need to start a new shell for the changes to take effect.`
+
+### For Bash
+
+```bash
+# Generates bash completion scripts
+echo "$(_MVT_IOS_COMPLETE=bash_source mvt-ios)" > ~/.mvt-ios-complete.bash &&
+echo "$(_MVT_ANDROID_COMPLETE=bash_source mvt-android)" > ~/.mvt-android-complete.bash
+
+# Sources the scripts in ~/.bashrc.
+. ~/.mvt-ios-complete.bash && .  ~/.mvt-android-complete.bash
+```
+
+### For Zsh
+
+```bash
+# Generates zsh completion scripts
+echo "$(_MVT_IOS_COMPLETE=zsh_source mvt-ios)" >  ~/.mvt-ios-complete.zsh &&
+echo "$(_MVT_ANDROID_COMPLETE=zsh_source mvt-android)" > ~/.mvt-android-complete.zsh
+
+# Sources the scripts in ~/.zshrc.
+.  ~/.mvt-ios-complete.zsh  && .  ~/.mvt-android-complete.zsh
+```
+
+For more information, visit the official [Click Docs](https://click.palletsprojects.com/en/stable/shell-completion/#enabling-completion).
+
+