Add note on officially supported platforms (#22344)

* Add initial section on support

* Update src/cpp/README.md

Co-Authored-By: Nicolas Noble <nicolasnoble@users.noreply.github.com>

* Reviewer comments

* Reviewer comments

Co-authored-by: Nicolas Noble <nicolasnoble@users.noreply.github.com>

src/cpp/README.md

@@ -9,6 +9,24 @@ This section describes how to add gRPC as a dependency to your C++ project.
 In the C++ world, there's no universally accepted standard for managing project dependencies.
 Therefore, gRPC supports several major build systems, which should satisfy most users.
 
+## Supported Platforms
+
+* Supported: These platforms are officially supported. We test our code on these
+platform and have automated continuous integration tests for them.
+
+* Best Effort: We do not have continous integration tests for these, but we are fairly confident that gRPC C++ would work on them.
+We will make our best effort to support them, and we welcome patches for such platforms, but we might need to declare bankruptcy on some issues.
+
+| Operating System | Architectures | Versions | Support Level |
+|------------------|---------------|----------|---------------|
+| Linux - Debian, Ubuntu, CentOS | x86, x64      | clang 3.4+, GCC 4.9+   | Supported |
+| Windows 10+                    | x86, x64      | Visual Studio 2015+    | Supported |
+| MacOS                          | x86, x64      | XCode 7.2+             | Supported |
+| Linux - Others                 | x86, x64      | clang 3.4+, GCC 4.9+   | Best Effort |
+| Linux                          | ARM           |                        | Best Effort |
+| iOS                            |               |                        | Best Effort |
+| Android                        |               |                        | Best Effort |
+
 ## Bazel
 
 Bazel is the primary build system used by the core gRPC development team. Bazel