Add language to README about branch naming

[cjcolvar]

Add the following branch renaming language to the README for this repository.

## Contributing 

If you're working on PR for this project, create a feature branch off of `main`. 

This repository follows the [Samvera Community Code of Conduct](https://samvera.atlassian.net/wiki/spaces/samvera/pages/405212316/Code+of+Conduct) and [language recommendations](https://github.com/samvera/maintenance/blob/master/templates/CONTRIBUTING.md#language).  Please ***do not*** create a branch called `master` for this repository or as part of your pull request; the branch will either need to be removed or renamed before it can be considered for inclusion in the code base and history of this repository.

Rationale

Git's default "master" branch derives from "master/slave" jargon which perpetuates systemic racist language and systems (see email Replacing "master" reference in git branch names). To uphold our Code of Conduct, we must move away from the term "master" in our technical language (as well as words like blacklist or whitelist).

Related work

• Background on the renaming effort is available in the working group notes.
• See the GitHub Renaming documentation for the changes that GitHub made during the renaming process.