Pull new version of protobuf sources.

third_party/protobuf/csharp/README.md

+This directory contains the C# Protocol Buffers runtime library.
+
+Warning: experimental!
+======================
+
+This code is still under significant churn. Unlike the original port,
+it only supports proto3 (but not *all* of proto3 yet) - there are no
+unknown fields or extensions, for example. protoc will (eventually)
+deliberately fail if it is asked to generate C# code for proto2
+messages other than descriptor.proto, which is still required for
+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.
+
+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.
+
+Supported platforms
+===================
+
+The runtime library is built as a portable class library, supporting:
+
+- .NET 4.5
+- Windows 8
+- Windows Phone Silverlight 8
+- Windows Phone 8.1
+- .NET Core (dnxcore)
+
+Building
+========
+
+Open the `src/Google.Protobuf.sln` solution in Visual Studio. Click "Build solution" to build the solution. You should be able to run the NUnit test from Test Explorer (you might need to install NUnit Visual Studio add-in).
+
+Supported Visual Studio versions are VS2013 (update 4) and VS2015. On Linux, you can also use Monodevelop 5.9 (older versions might work fine).
+
+History of C# protobufs
+=======================
+
+This subtree was originally imported from https://github.com/jskeet/protobuf-csharp-port
+and represents the latest development version of C# protobufs, that will now be developed
+and maintained by Google. All the development will be done in open, under this repository
+(https://github.com/google/protobuf).