Replace README.txt with README.md

Patch is working and marking it to RTBC.

If you are really interested in providing a patch and changing the README.txt to README.md (as you do in so many contrib modules), then please make an effort to update the most recent README.txt and also follow the Drupal guidelines → on README.md templates.
Just renaming the file to .md and adding a couple of headings is not sufficient.

Automatically closed - issue fixed for 2 weeks with no activity.