Update RxSwift.md
Update RxSwift.md

Add a note to better explain the need to manually retain the MoyaProvider.
ashfurrow/Moya,justinmakaila/Moya,Moya/Moya,Moya/Moya,justinmakaila/Moya,ashfurrow/Moya,Moya/Moya,justinmakaila/Moya