doc/go1.5: mention external linking support for windows

Change-Id: I2d2ea233f976aab3f356f9b508cdd246d5013e33
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7537

doc/go1.5.txt

@@ -41,6 +41,7 @@ testing/quick: support generation of arrays (https://golang.org/cl/3865)
 
 Tools:
 
+build: external linking support for windows (https://golang.org/cl/7163, 7282, 7283, 7284, 7534, 7535)
 cmd/go, go/build: add ${SRCDIR} variable expansion to cgo lines (https://golang.org/cl/1756)
 cmd/go: std wildcard now excludes commands in main repo (https://golang.org/cl/5550)
 cmd/go: .swig/.swigcxx files now require SWIG 3.0.6 or later