Add reminder about review in welcome msg

Author: apiraino

highfive/newpr.py

@@ -23,6 +23,8 @@
 raw_welcome = """Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from %s soon.
 
 Please see [the contribution instructions](%s) for more information.
+
+Tip: when your PR is ready for review, don't forget to fire up the command `@rustbot ready` to put your PR in the review queue of your assigned reviewer: this will help speed up the overall review process.
 """
 
 warning_summary = ':warning: **Warning** :warning:\n\n%s'