doc: add note about gccgo go go1.8.html

TBR=See https://golang.org/cl/33244

Updates #17929

Change-Id: I28559724322007d4259810c209a92ec1cc10f338
Reviewed-on: https://go-review.googlesource.com/33668Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.8.html

@@ -277,7 +277,13 @@ checked for Go pointers.
 
 <h3 id="gccgo">Gccgo</h3>
 
-<p>TODO</p>
+<p>
+Due to the alignment of Go's semiannual release schedule with GCC's
+annual release schedule,
+GCC release 6 contains the Go 1.6.1 version of gccgo.
+We expect that the next release, GCC 7, will contain the Go 1.8
+version of gccgo.
+</p>
 
 <h3 id="cmd_go">Go command</h3>