Closes: #1411

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)

The text was copied from https://code.google.com/p/simplesamlphp/wiki/LostState, corrected some language and added a piece on http/https stemming from the comments on that page.

Fixes #214