needs a composer.json

Problem/Motivation

When I try to install the dev version of the module with composer, I get

No valid composer.json was found in any branch or tag of https://git.drupalcode.org/project/flat_taxonomy, could not load a package from it.

Steps to reproduce

composer require drupal/flat_taxonomy:2.x-dev

Proposed resolution

add a composer.json file

Remaining tasks

make a fork and MR