Add bash v4+ requirement to the development guide

The development scripts require bash v4+. Today, this is not
documented in our development guide, and can cause to confusing
errors especially in MacOS (which has older versions).

This commit documents the bash v4+ requirement to help new
contributors who may have run into this issue.

Author: jerop

DEVELOPMENT.md

@@ -93,6 +93,10 @@ You must install these tools:
     --user="${USER}"
     ```
 
+1. [`bash`](https://www.gnu.org/software/bash/) v4 or higher: For scripts used to
+   generate code and update dependencies. On MacOS the default bash is too old,
+   you can use [Homebrew](https://brew.sh) to install a later version.
+
 ### Configure environment
 
 To [build, deploy and run your Tekton Objects with `ko`](#install-pipeline), you'll need to set these environment variables: