Add https://go-review.googlesource.com/c/43712,
"net/http/httputil: ReverseProxy should pass on unannounced Trailers"
to the relase notes.

Fixes #21307

Change-Id: I52c126987a5d0abc4153c0e71b535529c46cd457
Reviewed-on: https://go-review.googlesource.com/53290Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>