Document ActualBudget OpenID user-invite step

ActualBudget requires inviting additional OpenID users from its own
"Server Online" screen before their login is accepted, separate from
Authelia authenticating them successfully. Companion doc gets appended
to the generated README automatically (write_readme convention).

services/actualbudget.md

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+## Inviting additional OpenID users
+
+Enabling OpenID lets any Authelia user on the instance *authenticate*, but
+ActualBudget still gates actual access separately: only the account that
+completes the very first OpenID login becomes the server owner. Every other
+Authelia user has to be invited from inside ActualBudget before their login
+is accepted — Authelia successfully authenticating them isn't enough on its
+own, and the failure looks the same as a config problem if you don't know
+this step exists.
+
+To invite one:
+1. Log in as the owner account (the one that did the first OpenID login).
+2. Click the **"Server Online"** indicator — the user-management screen only
+   opens from there, not from the general Settings page, which is easy to
+   miss.
+3. Invite the new user by the **exact email address** set for them in
+   Authelia's `config/users.yml` — ActualBudget matches the OpenID identity
+   to an invited member by email, so any mismatch (typo, casing, different
+   domain) fails silently the same way as not being invited at all.
+4. Have them log in via OpenID again.