Restore tab completion docs to index

Author: danielbachhuber

index.md

@@ -91,10 +91,22 @@ WP-CLI project config:
 WP-CLI version: 0.23.0
 ```
 
+### Updating
+
 You can update WP-CLI with `wp cli update` ([doc](https://wp-cli.org/commands/cli/update/)), or by repeating the installation steps.
 
 Want to live life on the edge? Run `wp cli update --nightly` to use the latest nightly build of WP-CLI. The nightly build is more or less stable enough for you to use in your development environment, and always includes the latest and greatest WP-CLI features.
 
+### Tab completions
+
+WP-CLI also comes with a tab completion script for Bash and ZSH. Just download [wp-completion.bash](https://github.com/wp-cli/wp-cli/raw/master/utils/wp-completion.bash) and source it from `~/.bash_profile`:
+
+```
+source /FULL/PATH/TO/wp-completion.bash
+```
+
+Don't forget to run `source ~/.bash_profile` afterwards.
+
 ## Support
 
 WP-CLI's maintainers and project contributors do their best to respond to all new issues in a timely manner. To make the best use of their volunteered time, please first see if there may be an answer to your question in one of the following resources: