doc: add doc about nacl

[rsc]

need doc about nacl, maybe just a wiki page

[rsc]

Comment 1:

Labels changed: added release-go1.3.

[minux]

Comment 2:

Issue #7739 has been merged into this issue.

[bradfitz]

Comment 3:

Stub document is at http://golang.org/wiki/NativeClient

[rsc]

Comment 4:

Labels changed: added documentation.

[rsc]

Comment 5:

golang.org/wiki/NativeClient is filled out. Thanks to Dave Cheney for writing
misc/nacl/README.

Status changed to Done.