Minor README updates

(cherry picked from commit 172ad8c)

Author: michaelklishin

README.md

@@ -27,10 +27,16 @@ as archives.
 
 ## Supported Platforms and .NET Releases
 
+Future `6.x` versions of the library require .NET 4.6.2 or a .NET Core version implementing .NET Standard 2.0.
+
+### 5.x and 4.x
+
 `4.x` and `5.x` versions of the library require .NET 4.5.1 or later or .NET Core.
 For .NET Core users, 2.0 is the minimum supported version for `5.x` series.
 
-`3.6.x` releases support Linux and OS X on [Mono](http://www.mono-project.com/).
+### Older Versions
+
+`3.6.x` releases support Linux and MacOS on [Mono](https://www.mono-project.com/).
 
 ## Change Log