Replace README.txt with README.md

Thanks for your interest in this module.

However, the current README.txt is so incredibly small (there's very little to say) that the formatting at https://git.drupalcode.org/project/entity_reference_number_widget is just fine with plain text. If there was a lot to document, and we needed a bunch of different sections, etc, I'd consider reformatting this to md, but txt is all we need. The other changes in the MR for your proposed md conversion don't add anything but boilerplate text, and formatting we don't need. So I'm not going to merge this.

Thanks, anyway!
-Derek

dww → closed merge request !2