To ensure you have added a file to staging area, check status of the project again by:
git status
Notice there is new file added to be committed. Now that you have successfully added a file to staging area, you are supposed to commit the change that you have made with a message. Commiting is like keeping a log of changes made, which you can later go restore to. To commit the change, enter the following the command
git commit -m "add cat.txt"