Add compatibility doc (#2123)

Sources/GRPCCore/Documentation.docc/Articles/Compatibility.md

@@ -0,0 +1,22 @@
+# Compatibility
+
+Learn which versions of Swift are supported by gRPC Swift and how this changes
+over time.
+
+## Overview
+
+### Supported Swift Versions
+
+gRPC Swift supports the **three most recent Swift versions** that are **Swift 6
+or newer**. Within a minor Swift release only the latest patch version is supported.
+
+| grpc-swift | Minimum Supported Swift version |
+|------------|---------------------------------|
+| 2.0        | 6.0                             |
+
+### Platforms
+
+gRPC Swift aims to support the same platforms as Swift project. These are listed
+on [swift.org](https://www.swift.org/platform-support/).
+
+The only known unsupported platform from that list is currently Windows.

Sources/GRPCCore/Documentation.docc/Documentation.md

@@ -53,6 +53,10 @@ as tutorials.
 
 - <doc:Generating-stubs>
 
+### Project Information
+
+- <doc:Compatibility>
+
 ### Getting involved
 
 Resources for developers working on gRPC Swift: