doc: add httptest.ResponseRecorder note to go1.7.txt notes

Fixes #14928

Change-Id: Id772eb623815cb2bb3e49de68a916762345a9dc1
Reviewed-on: https://go-review.googlesource.com/21531Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.7.txt

@@ -16,6 +16,7 @@ API additions and behavior changes:
 
 runtime: add CallerFrames and Frames (CL 19869)
 testing/quick: now generates nil values (CL 16470)
+net/http/httptest: ResponseRecorder supports trailer (CL 20047) (compat impact: issue 14928)
 net/url: support query string without values (CL 19931)
 net/textproto: permit all valid token chars in CanonicalMIMEHeaderKey input (CL 18725)
 go/doc: add Unordered boolean to Example struct (CL 19280)