Use 'main' as default branch

This change switches to using main as the default branch to which
changes are pushed and from which releases are cut. The new name is
shorter, more neutral, and starting to get used by more and more
projects.

README.md

@@ -1,4 +1,4 @@
-[![pipeline](https://github.com/d-e-s-o/nitrocli/actions/workflows/.github/workflows/ci.yml/badge.svg)](https://github.com/d-e-s-o/nitrocli/commits/master)
+[![pipeline](https://github.com/d-e-s-o/nitrocli/actions/workflows/.github/workflows/ci.yml/badge.svg)](https://github.com/d-e-s-o/nitrocli/commits/main)
 [![crates.io](https://img.shields.io/crates/v/nitrocli.svg)](https://crates.io/crates/nitrocli)
 [![rustc](https://img.shields.io/badge/rustc-1.47+-blue.svg)](https://blog.rust-lang.org/2020/10/08/Rust-1.47.html)
 

doc/CONTRIBUTING.md

@@ -3,7 +3,7 @@ The following rules generally apply for pull requests and code changes:
 **Submit Pull Requests to the `devel` branch**
 
 The `devel` branch is where experimental features reside. After some
-soak time they may be ported over to `master` and a release will be cut
+soak time they may be ported over to `main` and a release will be cut
 that includes them.
 
 **Keep documentation up-to-date**