doc/go1.6.txt: start go1.6.txt with a note about nacl

Start go1.6.txt with a note that nacl ports are no longer
restricted to pepper_41 and a record of the text/template change.

Change-Id: I21dda64aec113c35caf1d565f29e3aac8171480a
Reviewed-on: https://go-review.googlesource.com/14004Reviewed-by: Rob Pike <r@golang.org>

doc/go1.6.txt

+Ports:
+
+NaCl is no longer restricted to pepper_41 (https://golang.org/cl/13958/)
+
+API additions and behavior changes:
+
+text/template: ExecError (https://golang.org/cl/13957/)