Readme.md: add note about why this repo is stale, and no longer necessary (if that's true)

[jaredthirsk]

I just discovered DynamicData and ReactiveUI (I know I am late to the party), wanted to use them, then found this repo but was disappointed to find it neglected.

I started going down the road of forking it and try porting or at least understanding it myself so I could salvage any useful concepts, only to hit a compile error and find ReactiveList was removed from ReactiveUI in favor of....DynamicData!

Here are some helpful links for other developers who might be on the same journey:

• https://www.reactiveui.net/blog/2018/08/deprecated-reactive-list
• https://github.com/reactiveui/ReactiveUI/issues/1372
• https://github.com/reactivemarbles/DynamicData/wiki/Introduction-for-ReactiveUI-users

[jaredthirsk]

Pull request: https://github.com/RolandPheasant/DynamicData.ReactiveUI/pull/9