Site /feed.xml is RSS 2.0, fix auto-discovery header

Refs #2996
Merge pull request #3176 from rud/bugfix/site-feed-is-rss
Update history to reflect merge of #3176 [ci skip]
Add appveyor configuration YAML.
Tweak appveyor.yml.
Tests should run now.
README.markdown: Add AppVeyor badge.