Update README.md

Author: kennykerr

README.md

@@ -11,6 +11,17 @@ C++/WinRT is an entirely standard C++ language projection for Windows Runtime (W
 
 C++/WinRT is part of the [xlang](https://github.com/microsoft/xlang) family of projects that help developers create APIs that can run on multiple platforms and be used with a variety of languages.
 
+# Building C++/WinRT
+
+Don't build C++/WinRT yourself - just download the latest version here: https://aka.ms/cppwinrt/nuget
+
+If you really want to build it yourself, the simplest way to do so is to run the `build_test_all.cmd` script in the root directory. Developers needing to work on the C++/WinRT compiler itself should go through the following steps to arrive at an efficient inner loop:
+
+* Open a dev command prompt pointing at the root of the repo.
+* Open the `cppwinrt.sln` solution.
+* Build the x64 Release configuration of the `cppwinrt` project only. Don’t attempt to build anything just yet.
+* Run `build_projection.cmd` in the dev command prompt.
+* Switch to the x64 Debug configuration in Visual Studio and build all projects as needed.
 
 # Contributing