add link to mdn-tests to README
bug 945117 - nav menu link for Add-ons zone
Merge pull request #1722 from groovecoder/addons-zone-link-945117

bug 945117 - nav menu link for Add-ons zone
Merge pull request #1721 from groovecoder/add-mdn-tests-to-README

add link to mdn-tests to README