Provide further instructions and examples for logging in

[shutupflanders]

What would you like to be added or enhanced
Mention in the docs that the username and password need to be base64 encoded when calling /login

Why is this needed
I was confused when I received an error attempting to login using my custom immudb credentials:

{
    "code": 3,
    "message": "illegal base64 data at input byte 4"
}

It led me to believe that my credentials were wrong, or they were possibly hardcoded in immugw, only when I thought to try a base64 decode on aW1tdWRi did I figure out the issue.