triagebot welcome message for new users

Author: RalfJung

triagebot.toml

@@ -17,6 +17,9 @@ allow-unauthenticated = [
 [assign]
 warn_non_default_branch = true
 contributing_url = "https://github.com/rust-lang/miri/blob/master/CONTRIBUTING.md"
+[assign.custom_welcome_messages]
+welcome-message = "Welcome, and thank you for contributing to Miri! To ensure a smooth reviewing experience, please read https://github.com/rust-lang/miri/blob/master/CONTRIBUTING.md#pull-review-process. In particular, please do not force-push to the PR branch except when you need to rebase due to a conflict or when the reviewer asks you for it."
+welcome-message-no-reviewer = "Welcome, and thank you for contributing to Miri! To ensure a smooth reviewing experience, please read https://github.com/rust-lang/miri/blob/master/CONTRIBUTING.md#pull-review-process. In particular, please do not force-push to the PR branch except when you need to rebase due to a conflict or when the reviewer asks you for it."
 
 [no-merges]
 exclude_titles = ["Rustup"]