Suggest debug-assertions when building with miri

Author: jyn514

CONTRIBUTING.md

@@ -146,6 +146,7 @@ git clone https://github.com/rust-lang/rust rustc
 cd rustc
 # Create a config.toml with defaults for working on miri.
 ./x.py setup compiler
+# Now edit `config.toml` and under `[rust]` set `debug-assertions = true`.
 
 # Build stage2 rustc using the stage1 compiler.
 # This step can take 30 minutes or more.