doc: go1.6.txt: note windows path/filepath.Join behaviour change

Change-Id: I321eba716319bf88695ac49580837b6254f1279e
Reviewed-on: https://go-review.googlesource.com/17474Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

doc/go1.6.txt

@@ -31,6 +31,7 @@ image: add NYCbCrA types (https://golang.org/cl/15671)
 math/rand: add Read (https://golang.org/cl/14522)
 net/http: HTTP/2.0 support (many CLs)
 net/url: make *url.Error implement net.Error (https://golang.org/cl/15672)
+path/filepath: Join("c:", "a") now returns `c:a` instead of `c:\a` on windows (https://golang.org/cl/17470)
 runtime: only one goroutine in traceback (https://golang.org/cl/16512) maybe
 strconv: QuoteToGraphic (https://golang.org/cl/14184/)
 text/template: ExecError (https://golang.org/cl/13957/)