Add blog post for 5.1.2

Author: dustinsoftware

site/jekyll/_posts/2019-11-24-5.1.2-release.md

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+---
+title: 'ReactJS.NET 5.1.2'
+layout: post
+author: Dustin Masters
+---
+
+🚀 We're pleased to announce the release of ReactJS.NET 5.1.2. A few small updates in this release:
+
+- .NET Core 3.0 support. The [tutorial project](https://github.com/reactjs/React.NET/tree/master/tutorial-code) has also been updated to demonstrate the (simple!) migration path.
+- React and ReactDOM updated to 16.12.0
+- Fix missing query parameter mapping for React.Router - thanks [@mattywong](https://github.com/mattywong)
+
+Please report any bugs in the [issue tracker](https://github.com/reactjs/React.NET/issues).
+
+Cheers!
+
+- [@dustinsoftware](https://twitter.com/dustinsoftware) and [@Daniel15](https://twitter.com/daniel15)