env/windows: note where OpenSSH for Windows came from
env/windows: note where OpenSSH for Windows came from

Change-Id: I4f006fc6726200be06a1e7c4f133b2c5a98af1ef
Reviewed-on: https://go-review.googlesource.com/52250
Reviewed-by: Jeff Johnson <e0aeede46a59107c4d834f64851ee491686571e7@google.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build