Add link to upgrade guide on Download or Release Notes pages

[andygunn]

There should probably be a link to the upgrade guide on the wiki:https://wiki.commotionwireless.net/doku.php?id=general_resources:documentation:router:upgrading_from_previous_versions - either on the Download page, or on the release notes page. Recommended text:

If you are updating from older versions of Commotion (Pre-Release, Developer Release, or 1.0 versions), we recommend reading over the Upgrade Guide on our wiki. It provides step-by-step instructions and a few important tips to make sure your upgrade goes smoothly.