Allow for public cloning of the repo including submodules

[kisp]

Public cloning of the repo currently fails. Let's make this work :)

[vagrant@arch ~]$ git clone --recursive https://github.com/alexgorji/musicscore.git
Cloning into 'musicscore'...
remote: Enumerating objects: 5488, done.
remote: Counting objects: 100% (933/933), done.
remote: Compressing objects: 100% (329/329), done.
remote: Total 5488 (delta 755), reused 711 (delta 598), pack-reused 4555
Receiving objects: 100% (5488/5488), 15.06 MiB | 11.26 MiB/s, done.
Resolving deltas: 100% (4241/4241), done.
Submodule 'musicxml' (git@github.com:alexgorji/musicxml.git) registered for path 'musicxml'
Cloning into '/home/vagrant/musicscore/musicxml'...
git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
fatal: clone of 'git@github.com:alexgorji/musicxml.git' into submodule path '/home/vagrant/musicscore/musicxml' failed
Failed to clone 'musicxml'. Retry scheduled
Cloning into '/home/vagrant/musicscore/musicxml'...
git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
fatal: clone of 'git@github.com:alexgorji/musicxml.git' into submodule path '/home/vagrant/musicscore/musicxml' failed
Failed to clone 'musicxml' a second time, aborting

[kisp]

I think we should pick one of the solutions here:

https://stackoverflow.com/questions/6031494/git-submodules-and-ssh-access

Specifically this one sounds good: https://stackoverflow.com/questions/40841882/automatically-access-git-submodules-via-ssh-or-https