Only show the contributing instructions for first timer with community reviews

[Urgau]

Only show the contributing instructions for first timer with community reviews like is done at

triagebot/src/handlers/assign.rs

Lines 323 to 327 in 847d964

	let mut welcome = messages::new_user_welcome_message(&assignee_text);
	if let Some(contrib) = &config.contributing_url {
	welcome.push_str("\n\n");
	welcome.push_str(&messages::contribution_message(contrib, &ctx.username));
	}

Otherwise every contributor even recurring one will have the contributing instructions: rust-lang/rust-clippy#17579 (comment)

[Kobzol]

Thanks!

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