We're renaming the default branch to "main"

[codesue]

Hello Model Card Toolkit contributors 👋

At the beginning of January, we will change the name of the default branch from master to main. This is consistent with the default branch name for newly-created GitHub repositories and for TFX Addons SIG projects. Open pull requests will automatically be re-targeted from master to main. Additionally, a notice will be displayed on the repository's homepage.

🚨 Action item for contributors: If you have a local clone of the repository, follow the instructions in the "What Your Teammates Have to Do" section of this guide to update your clone when the default branch is renamed.

For updates about this change, subscribe to #253.

[codesue]

Hi all, the renaming is complete, and the default branch is now main. Shoutout to @sklin93 driving this change. 🎉