Document MacOS/BSD differences for things like `grep -P`

[NicMcPhee]

People working on a Mac will probably need to use

• brew install coreutils
• brew install findutils
• brew install grep

and do all the associated PATH settings that brew suggests.

This would go in the little section about bash, etc., on different operating systems a few paragraphs into the "The basics of bash and Unix command-line tools" section.