README: clarify rules about golang.org/x/sys/windows contributions

Updates golang/go#77887

Change-Id: Ia804280123f2976ada91770eb0b00b6d0e3ef85e
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/769300
Commit-Queue: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>

README.md

diff --git a/README.md b/README.md
index 1d2278a..1733d36 100644
--- a/README.md
+++ b/README.md
@@ -88,7 +88,12 @@
 
 - Making API changes in internal packages, since those APIs are not publicly visible.
 - Making API or command-line changes in golang.org/x/build, since that is code to run the Go project, not for users to import and depend on.
-- Adding new system call numbers or direct system call wrappers (`//sys` lines) in golang.org/x/sys.
+- Adding new system call numbers or direct system call wrappers (`//sys` lines)
+  in golang.org/x/sys. Additions to x/sys/windows with a clear 1:1 mapping from
+  the Windows API to the Go API can be accepted without proposal review.
+  Determining whether a change is a 1:1 mapping is up to the golang.org/x/sys/windows
+  maintainers. Any additional APIs, including methods on types that aren't
+  clearly reflected in the Windows API, must go through Go proposal review.
 - Adding new C-equivalent data structures to support those system calls.
 
 Again, if in doubt, file a proposal.