add submodule

Add submodule to repository.

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

$ egit add-submodule <url> [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