Add mkdir config to instructions

[kristapsk]

Without this you get error:

$ npm run nsecbunkerd -- add --name "test"

> [email protected] nsecbunkerd
> node dist/index.js add --name test

nsecBunker uses a passphrase to encrypt your nsec when stored on-disk.
Every time you restart it, you will need to type in this password.

Enter a passphrase: ***
Enter the nsec for test: ***
Error writing config file: ENOENT: no such file or directory, open 'config/nsecbunker.json'