From a Terminal window:

$ mkcert -install
Created a new local CA at "/Users/kenny/Library/Application Support/mkcert" 💥
Password:

I'm pretty sure this is my Keychain password (the 'login' password for my user) but it's certainly not clear that that's what's being requested. Could this (or should) this be documented in the README? (I'd be willing to make a stab at doing so if it's considered helpful.)