add "Tests" section to contributing doc
add "Tests" section to contributing doc

realm/SwiftLint,victorpimentel/SwiftLint,victorpimentel/SwiftLint,Johnykutty/SwiftLint,lyft/SwiftLint,realm/SwiftLint,firebase/SwiftLint,victorpimentel/SwiftLint,jeffreybergier/SwiftLint,victorpimentel/SwiftLint,Johnykutty/SwiftLint,Johnykutty/SwiftLint,firebase/SwiftLint,jeffreybergier/SwiftLint,Johnykutty/SwiftLint,firebase/SwiftLint,lyft/SwiftLint,jeffreybergier/SwiftLint,realm/SwiftLint,jeffreybergier/SwiftLint,realm/SwiftLint,lyft/SwiftLint,firebase/SwiftLint,lyft/SwiftLint