When working with feature branches, the master branch on your fork becomes useless (you probably base your branches on upstream/master). As a result you fork's master gets outdated, and can lead to mistakes.
Deleting it is a good idea, but github needs a "default branch" to show on website, this script will create an orphan "_default" branch with just an explanatory README.md on it, and set that as default for github (you'll be asked your github password).
DISCLAIMER: This script will delete your master branch locally and remotely, be sure you don't need it anymore before running this!