PSA: Renaming the master branch to main

[dfed]

Terms like master are often associated with the master/slave metaphor, which is oppressive and can cause members of our community distress. In order to help all members of our community feel welcome contributing to and consuming this project, we will be renaming our master branch to main. Note that GitHub is aligned with this updated nomenclature. We will record the sha at which the branch name changed in our README, and we will re-target any open Pull Requests targeting master to instead target main.

Please note that contributors who have already cloned this repository will need to run the following command in order to continue contributing to this project: git fetch origin main && git checkout main && git branch -D master && git remote set-head origin main

We will make this change shortly. However, we will leave this issue open for six months in order to aid existing contributors to this repository.