Add ThirdPartyAssets/ as git submodule

[nucleartide]

...so as not to commit licensed assets to the public Breadforge project.

Workflow

The following commands add a new git submodule to the project:

git submodule add git@github.com:nucleartide/Breadforge-ThirdPartyAssets.git Assets/Plugins/ThirdPartyAssets
git add .
git commit -m "Blah blah"

You can then track the history of the submodule separately from the main repo.

However, note that if you change a git submodule's contents, you must make two commits:

1. You must commit the changes to the submodule first.
2. Then, you must commit the updated submodule reference to the parent repo.