Add cdn/ directory to repo for CDN auto-publishing

The cdn/ directory is automatically updated and committed as part of the
release process.

This enables CDNs such as jsdelivr to auto-publish the files under the
the current tag.

Fix #674

Signed-off-by: Byron Ruth <b@devel.io>
Merge pull request #684 from cbmi/issue-674

Add cdn/ directory to repo for CDN auto-publishing