Update C#-specific README

In particular, this removes the now-inaccurate instructions for building
Google.Protobuf targeting .NET 3.5.

csharp/README.md

@@ -16,18 +16,19 @@ To generate C# files from your `.proto` files, invoke `protoc` with the
 Supported platforms
 ===================
 
-The runtime library is built as a portable class library, supporting:
+The runtime library is built as a class library, supporting targets of:
 
-- .NET 4.5
-- Windows 8
-- Windows Phone Silverlight 8
-- Windows Phone 8.1
-- .NET Core
-- .NET 5
+- .NET 4.5+ (`net45`)
+- .NET Standard 1.1 and 2.0 (`netstandard1.1` and `netstandard2.0`)
+- .NET 5+ (`net50`)
 
 You should be able to use Protocol Buffers in Visual Studio 2012 and
 all later versions. This includes all code generated by `protoc`,
-which only uses features from C# 3 and earlier.
+which only uses features from C# 3 and earlier. When compiling generated
+code with old compilers (before C# 7.2) you need to define the
+`GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE` symbol in your project
+so that the generated classes don't implement `IBufferMessage`, which uses
+`ref struct` types.
 
 Building
 ========
@@ -42,12 +43,6 @@ in its implementation and runs tests under .NET 6. These features
 have no impact when using the compiled code - they're only relevant
 when building the `Google.Protobuf` assembly.
 
-In order to run and debug the AddressBook example in the IDE, you must
-install the optional component, ".Net Core 1.0 - 1.1 development tools
-for Web" (as it's labelled in current versions of the VS2017
-installer), above and beyond the main .NET Core cross-platform
-development feature.
-
 Testing
 =======
 
@@ -57,20 +52,11 @@ run using the Visual Studio Test Explorer or `dotnet test`.
 .NET 3.5
 ========
 
-We don't officially support .NET 3.5. However, there has been some effort
-to make enabling .NET 3.5 support relatively painless in case you require it.
-There's no guarantee that this will continue in the future, so rely on .NET
-3.5 support at your peril.
-
-To enable .NET 3.5 support, you must edit the `TargetFrameworks` elements of
-[src/Google.Protobuf/Google.Protobuf.csproj](src/Google.Protobuf/Google.Protobuf.csproj)
-(and [src/Google.Protobuf.Test/Google.Protobuf.Test.csproj](src/Google.Protobuf.Test/Google.Protobuf.Test.csproj)
-if you want to run the unit tests):
-
-Open the .csproj file in a text editor and simply add `net35` to the list of
-target frameworks, noting that the `TargetFrameworks` element appears twice in
-the file (once in the first `PropertyGroup` element, and again in the second
-`PropertyGroup` element, i.e., the one with the conditional).
+We don't support .NET 3.5. It *used* to be feasible to build this library
+targeting .NET 3.5, but a number of changes requiring newer runtime/framework
+features have been added over time. While it would no doubt be *possible* to
+rework the current implementation to allow most of the functionality to be built
+in .NET 3.5, this would create an undue maintenance burden.
 
 History of C# protobufs
 =======================
@@ -82,9 +68,9 @@ and maintained by Google. All the development will be done in open, under this r
 
 The previous project differs from this project in a number of ways:
 
-- The old code only supported proto2; the new code only supports
+- The old code only supported proto2; the new code initially only supported
 proto3 (so no unknown fields, no required/optional distinction, no
-extensions)
+extensions); since then proto2 support has been added
 - The old code was based on immutable message types and builders for
 them
 - The old code did not support maps or `oneof`