[NaCl Docs] add nacl_io page.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3634
R=eliben@chromium.org

Review URL: https://codereview.chromium.org/34053009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231431 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231431