doc/contrib.html: update links, mailing lists and link to 1.3 release
note.

https://code.google.com/p/go/source/browse/doc/contrib.html?r=de014d3583bbb3fa4056f3e40381fcff7b9a9738
Merge pull request #55 from gophersjp/doc/contrib.html

doc/contrib.html: update links, mailing lists and link to 1.3 release note.