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".

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

[RichardLitt]

@k88hudson I don't have permissions to do this, but I think it'd be a good idea.