Document how to verify encryption

git-crypt is made for encrypting sensitive files before they are pushed to a (possibly) public Git repository. However, when following the guide in the README exactly, there is no feedback that shows whether the intended files were actually encrypted.

A user can easily screw up the pattern and end up pushing sensitive files to a public repository.

I think it would be good if the instructions in the README included a note to check the output of `git crypt status -e` to make sure it includes all the intended files, as well as how to double check that none of those files were committed _before_ git crypt was installed.