Link to winapi and windows-sys crates

WinAPI provides low level libc-like functions but for Windows.
One may expect to find it here, so provide the links.

`winapi` crate works well, but it was last updated four years ago,
so it does not seem maintained.

`windows-sys` is provided by Microsoft. Official, looks good, but
I never used it, and it is somewhat harder to navigate.

So include links to both.

README.md

@@ -11,10 +11,15 @@ This crate exports all underlying platform types, functions, and constants under
 the crate root, so all items are accessible as `libc::foo`. The types and values
 of all the exported APIs match the platform that libc is compiled for.
 
+WinAPI bindings are not included in this crate. If you are looking for WinAPI
+bindings, consider using the [winapi] or [windows-sys] crates.
+
 More detailed information about the design of this library can be found in its
 [associated RFC][rfc].
 
 [rfc]: https://github.com/rust-lang/rfcs/blob/HEAD/text/1291-promote-libc.md
+[winapi]: https://docs.rs/winapi
+[windows-sys]: https://docs.rs/windows-sys
 
 ## v1.0 Roadmap