> Where git-crypt really shines is where most of your repo is public, but you have a few files (perhaps private keys named *.key, or a file with API credentials) which you need to encrypt.

Awesome, this is just my use case. I had been not sure if this was actually the use case you were focusing on -- I'd suggest maybe including that exact above sentence in the README, so potential users can understand what you are intending the tool for.