Require \uXXXX for non ASCII

@stephentoub

Per discussion in https://github.com/dotnet/corefx/issues/77

Commit migrated from https://github.com/dotnet/corefx/commit/fbe56f762e0aa80dafde14b013958c890c5033ae

docs/libraries/coding-guidelines/coding-style.md

@@ -28,6 +28,7 @@ The general rule we follow is "use Visual Studio defaults".
 12. We use PascalCasing to name all our constant local variables and fields. The only exception is for interop code where the constant value should exactly match the name and value of the code you are calling via interop.
 13. We use ```nameof(...)``` instead of ```"..."``` whenever possible and relevant.
 14. Fields should be specified at the top within type declarations.
+15. When including non-ASCII characters in the source code use Unicode escape sequences (\uXXXX) instead of literal characters. Literal non-ASCII characters occasionally get garbled by a tool or editor.
 
 We have provided a Visual Studio 2013 vssettings file (`corefx.vssettings`) at the root of the corefx repository, enabling C# auto-formatting conforming to the above guidelines. Note that rules 7 and 8 are not covered by the vssettings, since these are not rules currently supported by VS formatting.