Replace README.txt with README.md

Issue still exists in the patch.

git.drupalcode.org_project_image_webp_toolkit_-_merge_requests_3.diff.txt:18: trailing whitespace.
This module gives the possibility to transform an image whatever the type into
warning: 1 line adds whitespace errors.

Updated the MR to address the issue in comment #5 and follow the README.md template → .

Please review.
Thank You.

Patch is working and marking it to RTBC.

Configuration section remaining. Keeping it to NW status.