Add to README authorizations notes

[snetAT]

I run an ntfy server myself.
For the whole thing to work you have to set permissions for the topic on the ntfy server, for example

ntfy access everyone "up*" read-write

[rfc2822]

It's not really related to this repo, but rather about how to use ntfy, right?

We can add it to the README though. Can you create a PR for the README?