README installation instructions (Combine, RxSwift and ReactiveSwift package variants)

[haifengkao]

please check Package.swift

[haifengkao]

my bad. I should have import BridgeMiddlewareCombine instead of BridgeMiddlewareCombine

[luizmb]

Hi @haifengkao, I'm a bit busy these days, so that's why I couldn't reply.
Could you please open a PR with README giving installation instructions? That would be great, otherwise let's keep this issue open to keep track on that.