doc: add cmd/trace to go1.5.txt

Change-Id: I4d35a96ace71b68a91561c08fd7331859d9dd19d
Reviewed-on: https://go-review.googlesource.com/5593Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

doc/go1.5.txt

@@ -46,6 +46,7 @@ cmd/go: std wildcard now excludes commands in main repo (https://golang.org/cl/5
 cmd/go: .swig/.swigcxx files now require SWIG 3.0.6 or later
 cmd/vet: better validation of struct tags (https://golang.org/cl/2685)
 cmd/ld: no longer record build timestamp in Windows PE file header (https://golang.org/cl/3740)
+cmd/trace: new command to view traces (https://golang.org/cl/3601)
 
 Performance: