Move `master` branch to `main` about sloop

[jennybc]

I have moved the default branch from master to main here (and elsewhere)!

This blog post announces the larger effort and explains how contributors can update their local environments:

Renaming the default branch

Here's the TL;DR:

1. Update usethis (you want usethis v2.1.2 or higher):

   install.packages("usethis")

2. Anyone with an existing fork or clone can adjust to the renamed default branch with:

   usethis::git_default_branch_rediscover()

   If you wish, you can read the documentation for git_default_branch_rediscover().
   Or do the command line equivalent (see the blog post).

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