Add windows_sys readme

Author: ChrisDenton

library/std/src/sys/pal/windows/c/README.md

@@ -0,0 +1,9 @@
+The `windows_sys.rs` file is autogenerated from `bindings.txt` and must not
+be edited manually.
+
+To add bindings, edit `bindings.txt` then regenerate using the following command:
+
+    ./x run generate-windows-sys && ./x fmt library/std
+
+If you need to override generated functions or types then add them to
+`library/std/src/sys/pal/windows/c.rs`.