[README] XCTest has type safety like Quick

It has the exact same behaviour as Quick.

```swift
func XCTAssertEqual<T : Equatable>(expression1: @autoclosure () -> [T], expression2: @autoclosure () -> [T], _ message: String = default, file: String = default, line: UInt = default)
```

> Nimble uses generics--only available in Swift--to ensure type correctness. That means type checking is not available when using Nimble in Objective-C