Describe how to invoke the AFL fuzz tester on the KeePassXC
CLI tool. As suggested in #2729.
Fuzz test build of keepassxc-cli takes database password from
environment variable instead of requiring it to be empty.
Provide two empty kdbx files as initial fuzzer input, one
kdbx 3 and one kdbx 4, both with minimal number of decryption
rounds to speed up the test.
