Writing an introduction for the contribution guide (#161)

Co-authored-by: Takashi Idobe <idobetakashi@gmail.com>

CONTRIBUTING.md

@@ -1,5 +1,15 @@
 # Contributing
 
+## Introduction
+
+This book is a catalogue of Rust programming techniques, (anti-)patterns, idioms and other explanations.
+It is a compilation of collective (sometimes implicit) knowledge as well as experiences that have emerged through collaborative work.
+
+The patterns described here are __not rules__, but should be taken as guidelines for writing idiomatic code in Rust.
+We are collecting Rust patterns in this book so people can learn the tradeoffs between Rust idioms and use them properly in their own code.
+
+If you want to be part of this effort here are some ways you can participate:
+
 ## Discussion board
 
 If you have a question or an idea regarding certain content but you want to have feedback of fellow community members