Move `master` branch to `main`

The master branch of this repository will soon be renamed to main, as part of a coordinated change across several GitHub organizations (including, but not limited to: tidyverse, r-lib, tidymodels, and sol-eng). We anticipate this will happen by the end of September 2021.

That will be preceded by a release of the usethis package, which will gain some functionality around detecting and adapting to a renamed default branch. There will also be a blog post at the time of this master --> main change.

The purpose of this issue is to:

Help us firm up the list of targetted repositories
Make sure all maintainers are aware of what's coming
Give us an issue to close when the job is done
Give us a place to put advice for collaborators re: how to adapt

message id: euphoric_snowdog

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

When I ran usethis::git_default_branch_rename(), I saw this output, which might require some follow-up:

✓ Default branch of the source repo 'r-lib/remotes' has moved: 'master' --> 'main'
• Be sure to update files that refer to the default branch by name.
  Consider searching within your project for 'master'.
x These GitHub Action files don't mention the new default branch 'main':
  - '.github/workflows/pkgdown.yaml'
  - '.github/workflows/R-CMD-check.yaml'
  - '.github/workflows/test-coverage.yaml'
x Some badges may refer to the old default branch 'master':
  - 'README.md'
✓ Default branch of local repo has moved: 'master' --> 'main'

As a result, I won't close this issue, but rest assured that the default branch has been renamed.

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:

Update usethis (you want usethis v2.1.2 or higher):
```
install.packages("usethis")
```
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).

message id: goofy_chipmunk

r-lib / remotes

Move `master` branch to `main` #648