Use dynamic spec badge to keep in sync with docs

The docs site will likely evolve independently of the main readme, so make it stay in auto-sync with the new version.json file we emit for the spec.

readme.md

@@ -3,7 +3,8 @@
 Core specification and reference implementation for integrating GitHub Sponsors into 
 libraries and tools.
 
-[![Spec](https://img.shields.io/badge/spec-2.0.0-black?labelColor=EA4AAA)](https://www.devlooped.com/SponsorLink/spec/)
+[![Spec](https://img.shields.io/badge/dynamic/json?url=https%3A%2F%2Fwww.devlooped.com%2FSponsorLink%2Fspec%2Fversion.json&query=%24.version&label=spec&labelColor=EA4AAA&color=black
+)](https://www.devlooped.com/SponsorLink/spec/)
 [![Version](https://img.shields.io/nuget/v/dotnet-sponsor.svg?color=royalblue)](https://www.nuget.org/packages/dotnet-sponsor) 
 [![Downloads](https://img.shields.io/nuget/dt/dotnet-sponsor.svg?color=green)](https://www.nuget.org/packages/dotnet-sponsor)