How to start tracking directories/files again which were removed from git index?

Using below command i removed few directories from git index:

git rm -r --cached <your directory>

Now how can i add those untracked directories to git index again?

Just add them again to your index using git add:

git add <your directory>

Note that this will add all files, and not only those that were removed with your previous command. There is no way to only undo your command.