Update README.md for C#

csharp/README.md

 This directory contains the C# Protocol Buffers runtime library.
 
-Warning: experimental!
-======================
+Status: Alpha - ready for early adopters
+========================================
 
 This code is still under significant churn. Unlike the original port,
 it only supports proto3 (but not *all* of proto3 yet) - there are no
@@ -12,15 +12,20 @@ reflection. (It's currently exposed publicly, but won't be
 eventually.)
 
 Also unlike the original port, the new version embraces mutability -
-there are no builder types. We plan to add "freezing" operations as
-well as cloning, however.
+there are no builder types.
 
 Usage
 =====
 
-Use `protoc` with the `--csharp_out` option to generate C# files in the specified directory.
-Include these in your C# project, and add a reference to the `Google.Protobuf` project. Currently
-there is no NuGet package for this, but we will be building one as soon as the API is stable.
+The easiest way how to use C# protobufs is via the `Google.Protobuf`
+NuGet package. Just add the NuGet package to your VS project.
+
+Besides C# runtime library, the NuGet package also contains 
+precompiled version of `protoc.exe` and a copy of well known `.proto`
+files under the package's `tools` directory.
+
+To generate C# files from your `.proto` files, invoke `protoc` with the 
+`--csharp_out` option.
 
 Supported platforms
 ===================