Please add information about INWX-account-security to Readme

[dotbanana]

This is from Readme of https://github.com/oGGy990/certbot-dns-inwx.

It is recommended to create a subaccount in the INWX interface and restrict this account to the 'DNS management' role. This prevents the possible loss of your domains in case your account credentials are stolen.
The shared secret is your INWX 2FA OTP key. It is shown to you when setting up the 2FA. It is not the 6 digit code you need to enter when siging in. If you are not using 2FA, simply keep the value the way it is.

I was about to give up on the concept of this because i thought i have to give my real account credentials into this.
When i checked out other implementations out of curiosity i found this.
I think it would be extremely useful to notice people to also here.

[jemus42]

Just wanted to chime in in say that I was really happy to see this issue before I set up caddy with inwx yesterday. Both the note about the service account for DNS management and what to actually enter as shared secret where things I would have liked to have in the plugin docs, with the latter being kind of a big one!

[ihmels]

The reason why this plugin or libdns/inwx supports 2FA is that, in the past, it wasn't possible to create additional accounts with INWX. As a result, users would have had to disable 2FA on their main account. This is no longer the case.

Because of this, I'm now considering removing 2FA support. From a security standpoint, it no longer makes sense: both the password and the 2FA shared secret are stored together in the same place, which defeats the purpose of two-factor authentication.