Add submodule to repository.

Similar: [add-submodule], [init-submobules], [commit-push-submobules], [amend-push-submobules].

$ egit add-submodule  [dir]
url: short/full repository URL
dir: name of target directory

Clone github wiki as submodule in directory "wiki"
$ git clone https://github.com/nodef/extra-git
$ cd extra-git
$ egit add-submodule nodef/extra-git.wiki wiki

[add-submodule]: https://github.com/nodef/extra-git/wiki/add-submodule
[init-submobules]: https://github.com/nodef/extra-git/wiki/init-submobules
[commit-push-submobules]: https://github.com/nodef/extra-git/wiki/commit-push-submobules
[amend-push-submobules]: https://github.com/nodef/extra-git/wiki/amend-push-submobules