Rework README.md using markdown syntax

This is a small improvement, so that the file is more easily scanned, and it tidies up the formatting a little in a few places.

I did not take the time to make the file fit the new template ( https://www.drupal.org/docs/develop/managing-a-drupalorg-theme-module-or... → ), if you want me to do that I'm happy to do the work.

Thanks @DamienMcKenna, I'll update it to match the README.md template.

I reworked some bits to more closely match the README template. I created the MR in Gitlab, for easier preview.

Looks good, thanks Ressa.

Updated the file as per the suggestions in #7.
Kindly review the changes.

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