Replace README.txt with README.md

As per md format updated the readme file

Please review it!

Sonal Gyanani → made their first commit to this issue’s fork.

Issue exists in the patch.

git.drupalcode.org_project_sula_calculator_-_merge_requests_1.diff.txt:55: trailing whitespace.
- Customize the module settings in
warning: 1 line adds whitespace errors.

Updated the MR to address the issue in comment #5.

Please review.
Thank You.

The patch seems to be working fine, Moving it to RTBC.