README: add links to package, manuals, and GitHub
GitHub now supports relative links from the README

https://help.github.com/articles/relative-links-in-readmes
bootstrap CSS failed for HTML manuals under HTTPS

See also https://github.com/sunaku/readably/pull/3
drop redundant nil check in normal_text => escape
Merge branch 'other'
Merge branch 'patch'
Version 2.1.1 (2014-06-21)
upgrade to Redcarpet 3.1 for auto heading anchors
Redcapret breaks compatibility in minor releases