README.md: Add link to Homebrew blog

Inspired by https://github.com/Homebrew/brew/issues/8096 and https://github.com/Homebrew/brew/pull/8098. I think the blog is an important way for regular users to keep up-to-date with Homebrew evolution, so it deserves an entry in the main docs page.

Update docs/README.md

Co-authored-by: Mike McQuaid <mike@mikemcquaid.com>