Document that rebasing feature branches is preferred
Document that rebasing feature branches is preferred

over merging master. This is already enforced in the Dangerfile,
as of #1775 but we should probably document it.

lyft/SwiftLint,realm/SwiftLint,lyft/SwiftLint,lyft/SwiftLint,lyft/SwiftLint,realm/SwiftLint,realm/SwiftLint,realm/SwiftLint