Blog: add announcement that rnp now available at Homebrew

[ronaldtse]

Since we have the formula in homebrew-core now, there no need to keep this repo active (rnpgp/homebrew-rnp#41)

brew install rnpgp/rnp/rnp installs the homebrew-core version of the formula now due to migration added in tap_migrations.json

Or course brew install rnp works too

Originally posted by @alex-sc in #39