Renaming the default branch from `master` to `main`

Git and GitHub plan to rename the default branch from `master` to `main`.

So it would be more convenient to rename our default branch name to `main` for those who visit here.
Also, it is consistent with our README where default branch names are shown as `main`.

The repository owner can rename it as described in "[Renaming existing branches - GitHub/renaming](https://github.com/github/renaming#renaming-existing-branches)".

I appreciate it if you could give it a kind consideration.
Thank you so much for an amazing project.

cf: [Git 2.30 Released As More Projects Shift To "Main" As Their Default Branch Name](https://www.phoronix.com/scan.php?page=news_item&px=Git-2.30-Released)

Start with the repository's README and the GitHub renaming guidance linked in the issue. Confirm whether the default branch can be renamed by the repository owner and check for references that still use `master`; done means the default branch is `main` and the documented branch name is consistent.

Written by the indexing model from the issue text.