Add composer.json for better integration with METSIS Drupal

[magnarem]

Adding a composer.json file to the project root of this mmd project, will make it more streamlined install and version control of mmd for metsi_ drupal (xslt).

I will try to add a composer.json file and test if it works as expected.

[magnarem]

I have added to branch 3.3.5 (as composer did not like my issue-335 name, as it needs some strict versioning for branches and tags).

I can confirm that just adding this composer.json to the root of this repo, make the satis composer server and the composer client to be able to install the mmd package in my web vendor directory.

Creating an MR and then this issue can be closed when merged.

[ferrighi]

Included with #340 . Closing.