doc: add SIGPIPE behaviour change to go1.8.txt

CL 32796 changes the SIGPIPE behaviour for c-archive and c-shared
programs. Add it to go1.8.txt.

Change-Id: I31200187033349c642965a4bb077bcc77d5329a3
Reviewed-on: https://go-review.googlesource.com/33397Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.8.txt

@@ -256,6 +256,7 @@ runtime: sleep on CLOCK_MONOTONIC in futexsleep1 on freebsd (CL 30154)
 runtime: use RtlGenRandom instead of CryptGenRandom (CL 29700)
 runtime: use clock_gettime(CLOCK_REALTIME) for nanosecond-precision time.now on arm64, mips64x (CL 32177)
 runtime: use correct system page size on all arches (CL 25022)
+runtime: handle SIGPIPE in c-archive and c-shared programs (CL 32796)
 
 syscall: Clearenv now unsets env vars on Windows (CL 33168)
 syscall: add bounds checking and error returns to ParseNetlinkMessage (CL 26990)