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@3383 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3382 44740490-163a-0410-bde0-09ae8108e29a