Document supported runtimes (dotnet#182)

* Document supported runtimes

* More kestrel improvements

Author: Tratcher

docs/docfx/articles/runtimes.md

@@ -0,0 +1,21 @@
+---
+uid: runtimes
+title: Supported Runtimes
+---
+
+# YARP Supported Runtimes
+
+YARP supports ASP.NET Core 3.1 and 5.0.0 Preview 4 or later. You can download the .NET 5 Preview 4 SDK from https://dotnet.microsoft.com/download/dotnet/5.0.
+
+YARP will be taking advantage of 5.0.0 features and optimizations as they become available. This does mean that some features may not be available if you're running on 3.1.
+
+## Related 5.0.0 Runtime Improvements
+
+These are related improvements in .NET or ASP.NET Core 5.0.0 that YARP is able to take advantage of. We expect to add more as they become available.
+- Kestrel [reloadable config](https://github.com/dotnet/aspnetcore/issues/19376).
+- Kestrel HTTP/2 performance improvements.
+  - [HPACK static compression](https://github.com/dotnet/aspnetcore/pull/20058)
+  - [HPACK dynamic compression](https://github.com/dotnet/aspnetcore/pull/19521).
+  - [Allocation savings via stream pooling](https://github.com/dotnet/aspnetcore/pull/18601)
+  - [Allocation savings via pipe pooling](https://github.com/dotnet/aspnetcore/pull/19356)
+- HttpClient HTTP/2 [performance improvements](https://github.com/dotnet/runtime/issues/35184).

docs/docfx/articles/toc.yml

@@ -1,2 +1,4 @@
 - name: Getting Started
   href: getting_started.md
+- name: Supported Runtimes
+  href: runtimes.md