Replace README.txt with README.md

Unable to apply the patch.
error: README.txt: No such file or directory

Readme.md files is there need to update as per Drupal recommendation.

Renaming the file is not sufficient. See README.md template → for what that file should contain.

The project already has a correct README.md file. It has been committed one year ago.

There is eventually a slight change that should be done on that file, but a MR that renames a file that does not exist is not a correct MR.