Change the extension of the documentation files from .txt to .md so that they can processed as markdown and displayed in github.

(being respectful with occurences that might change the behaviour, i.e. default database prefixes)

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2872 44740490-163a-0410-bde0-09ae8108e29a