README.md: Add information to convert Mercurial to Git.
README.md: Improve Mercurial to Git conversion description.
Add release-update-versions.zsh to simplify release process
Doc,FAQ,Intro: update for 5.5.1
Add release notes for 5.5.1
Arc/source: use HTTPS for sourceforge download links
Doc,FAQ,Intro: update for 5.5
Add release notes for 5.5