doc: mention Go requires OpenBSD 5.9 or above in go1.8.html

Change-Id: Ia19e4d2f319002b5b26e50258436f5c4bd281cc7
Reviewed-on: https://go-review.googlesource.com/34113Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.8.html

@@ -86,6 +86,10 @@ have firmware that doesn't enable kernel FPU emulation; Go won't run on such mac
 On DragonFly BSD, Go now requires DragonFly 4.4.4 or later. <!-- CL 29491, CL 29971 -->
 </p>
 
+<p>
+On OpenBSD, Go now requires OpenBSD 5.9 or later. <!-- CL 34093 -->
+</p>
+
 <p>
 The Plan 9 port's networking support is now much more complete
 and matches the behavior of Unix and Windows with respect to deadlines