[docs] Add RSS feed.xml for blog posts

uses `feed.xml` from https://github.com/snaptortoise/jekyll-rss-feeds
(slightly modified for `site.url` + `site.baseurl`)
[docs] Use date_to_xmlschema in feed.xml
[docs] RSS: Pass title through xml_escape
Merge pull request #49 from paulshen/jekyllrss

[docs] Add RSS feed.xml for blog posts