* Update references to `master` on documentation
The master branch has been renamed to main, but the docs
still reference it as master.
* Update the changelog URI
The changelog URI should be pointed to the main branch, where it is
updated. The master branch is archived and has not been updated since
version 5.0.0.
* Update the main branch on Github Actions config
Since shoulda-matchers 5.1.0, the main branch of the repository
is `main`.
* Move section on documentation from README to CONTRIBUTING
* Add a setup script that contributors and maintainers can use to set up
a dev environment
[skip ci]