move CONTRIBUTING.md to rustc-dev-guide

Author: mark-i-m

src/SUMMARY.md

@@ -28,6 +28,7 @@
 
 # Contributing to Rust
 
+- [Introduction](./contributing.md)
 - [About the compiler team](./compiler-team.md)
 - [Walkthrough: a typical contribution](./walkthrough.md)
 - [Bug Fix Procedure](./bug-fix-procedure.md)