rename master branch to main

[nigelmegitt]

There's been a lot of news recently about power relationships and how their terminology has been adopted into code unthinkingly. One of those is the use of the term "master"; a lot of projects have been renaming their main branch from "master" to something else, like "main". For example see https://tools.ietf.org/id/draft-knodel-terminology-02.html#rfc.section.1.1.1

Proposal is to rename master branch to main.