Mention prebuild in readme as it is needed by the cppwinrt project (#805)

Author: asklar

README.md

@@ -17,7 +17,7 @@ If you really want to build it yourself, the simplest way to do so is to run the
 
 * 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.
+* Build the x64 Release configuration of the `prebuild` and `cppwinrt` projects only. Do not attempt to build anything else 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.