Docs/non root user with systemd

[boenshao]

Related to #186 (comment).

Quote me,

A typical use case is storing configurations and tokens under /etc/rathole/xxx.toml with permission set to 0600, that is, only the root user (or owner) can read the file. Preventing arbitrary users on the system from accessing the token. Yet with DynamicUser=yes the rathole process spawned by systemd won't have the privilege to read the file either.

This PR update the document with a more comprehensive systemd setup without root privilege.

[rapiz1]

Good work! I may be nitpicking but there's an unnecessary change.