Update README.md for release 0.33.1

Author: mluisbrown

README.md

@@ -4,7 +4,11 @@
 [![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Ftrading-point%2Freactiveswift-composable-architecture%2Fbadge%3Ftype%3Dswift-versions)](https://swiftpackageindex.com/trading-point/reactiveswift-composable-architecture)
 [![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Ftrading-point%2Freactiveswift-composable-architecture%2Fbadge%3Ftype%3Dplatforms)](https://swiftpackageindex.com/trading-point/reactiveswift-composable-architecture)
 
-[Point-Free's](https://github.com/pointfreeco) [The Composable Architecture](https://github.com/pointfreeco/swift-composable-architecture) uses Apple's Combine framework as the basis of its `Effect` type. Unfortunately, Combine is only available on iOS 13 and macOS 10.15 and above. In order to be able to use it with earlier versions of the OSes, this fork has adapted The Composable Architecture to use [Reactive Swift](https://github.com/ReactiveCocoa/ReactiveSwift) as the basis for the `Effect` type. Since release 0.8.1 this fork has also added Linux compatibility.
+[Point-Free's](https://github.com/pointfreeco) [The Composable Architecture](https://github.com/pointfreeco/swift-composable-architecture) uses Apple's Combine framework as the basis of its `Effect` type.
+
+This fork has adapted The Composable Architecture to use [Reactive Swift](https://github.com/ReactiveCocoa/ReactiveSwift) as the basis for the `Effect` type. The original motivation for this was to allow using The Composable Architecture in OS versions prior to iOS 13 and macOS 10.15, which are the earliest versions that support Combine and, therefore, TCA. Linux compatibility was also added since release 0.8.1.
+
+However, [for a number of reasons](http://michael-brown.net/2021/the-future-of-the-reactiveswift-composable-architecture/), maintaining support for older Apple OSes has become impractical, so as of release 0.33.1, the minimum OS requirements are the same as that of TCA: iOS 13 and macOS 10.15. Releases 0.28.1 and earlier of course still support older OS versions.
 
 ## Effect type implementations
 <details>