Link to the RDFa codelab

Per Gregg Kellogg's suggestion at
http://lists.w3.org/Archives/Public/public-rdfa/2014Apr/0011.html, add a link
to the RDFa/schema.org codelab from the rdfa.info/docs page.

Signed-off-by: Dan Scott <dan@coffeecode.net>
Merge pull request #42 from dbs/rdfa_codelab

Link to the RDFa codelab
Merge pull request #4 from rdfa/master

Upstream changes