BUILDING.md

New swift-protobuf release:

• brew upgrade swift-protobuf
• make protobuf

New protocol release:

• Update sass to the new tag
• make protobuf
• Update Versions.minProtocolVersion
• Update README 'Requirements'

New dart_sass release:

• Update VERSION_DART_SASS
• Update README Sass badge
• make dart_sass
• Figure corresponding sass/sass release, update that tag
• make deprecations and check Deprecations.swift

New swift-sass release:

• swift package update & Xcode -> File -> Swift Packages -> Update...
• .jazzy.yaml - two places
• Update README 'Installation'
• Rebuild docs
• Update changelog
• Commit and tag
• Github release