triagebot welcome message for new users

Author: RalfJung

CONTRIBUTING.md

@@ -13,7 +13,7 @@ for a list of Miri maintainers.
 
 [Rust Zulip]: https://rust-lang.zulipchat.com
 
-### Pull review process
+### PR review process
 
 When you get a review, please take care of the requested changes in new commits. Do not amend
 existing commits. Generally avoid force-pushing. The only time you should force push is when there

triagebot.toml

@@ -17,6 +17,17 @@ 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#pr-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#pr-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"]