Add bash completion for gaiad and gaiacli (#3334)

docs/gaia/gaiacli.md

@@ -761,3 +761,36 @@ The transaction can now be sent to the node:
 ```bash
 gaiacli tx broadcast signedTx.json
 ```
+
+## Shells completion scripts
+
+Completion scripts for popular UNIX shell interpreters such as `Bash` and `Zsh`
+can be generated through the `completion` command, which is available for both
+`gaiad` and `gaiacli`.
+
+If you want to generate `Bash` completion scripts run the following command:
+
+```bash
+gaiad completion > gaiad_completion
+gaiacli completion > gaiacli_completion
+```
+
+If you want to generate `Zsh` completion scripts run the following command:
+
+```bash
+gaiad completion --zsh > gaiad_completion
+gaiacli completion --zsh > gaiacli_completion
+```
+
+::: tip Note
+On most UNIX systems, such scripts may be loaded in `.bashrc` or
+`.bash_profile` to enable Bash autocompletion:
+
+```bash
+echo '. gaiad_completion' >> ~/.bashrc
+echo '. gaiacli_completion' >> ~/.bashrc
+```
+
+Refer to the user's manual of your interpreter provided by your
+operating system for information on how to enable shell autocompletion.
+:::