Update documentation

- Update README and ChangeLog and projectile-git-submodule-command
docstring.
Minor fix for retrieving submodules files

Use file-name-as-directory to convert a file to a directory, since it
is portable. The current implementation only works in Linux, as that's
exactly what file-name-as-directory does when it's in Linux.
Merge pull request #471 from tuhdo/master

Update documentaiton and minor fix for retrieving submodules files