doc: update go1.6.txt

Change-Id: I1164c7a76cf6e6c48ca5153d4c163f5962c4f0cd
Reviewed-on: https://go-review.googlesource.com/17622Reviewed-by: Dave Cheney <dave@cheney.net>

doc/go1.6.txt

@@ -33,6 +33,7 @@ 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
+runtime: best-effort detection of concurrent misuse of maps (https://golang.org/cl/17501)
 strconv: QuoteToGraphic (https://golang.org/cl/14184/)
 text/template: ExecError (https://golang.org/cl/13957/)
 text/template: trimming spaces (https://golang.org/cl/14391/)