Add enslavement references in GitHub language to Ethical considerations

[kallewesterling]

See articles like:

• "GitHub to replace "master" with alternative term to avoid slavery references", ZDNet, June 14, 2020.
• "Renaming the default branch from master", GitHub — their own instructions, where they also remind users that later this year, they will "make it seamless for existing repositories to rename their default branch."

Here are tutorials:

• "How To: Safely Rename master Branch on GitHub ✊🏾", Jake Jarvis (Blog), June 28, 2020.