RFC - Add documentation for strings and Unicode issues

[jbarlow83]

I requested this in #591. Thanks to clarifications from @jagerman in that issue I felt I had gather enough information to contribute this myself. Comments welcome.

[jbarlow83]

I concede that I am not personally interested in anything to do with wide characters and welcome expansion to that section if anyone feels a need.

[wjakob]

I think this looks great, but a few words about wstring and wchar_t should indeed be added to remark on the fact that these are also supported. All that needs to be said is that these do the proper thing analogous to u16string, etc., but what that is specifically depends on the platform. wstring and wchar_t are particularly important on Windows since the native WIN32 API makes frequent use of strings using the same encoding. So if you want to wrap a native WIN32 function, you'll want to use these.

[jbarlow83]

Okay, I modified a simple Win32 example from a Microsoft article and some details about characters literals.

[wjakob]

This is great -- thank you for such an in-depth writeup!