add a separate CONTRIBUTING document

Author: yaperez-anssi

CONTRIBUTING.md

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+# Contributing to This Project
+
+Thank you for your interest in this project. Please read this document
+carefully before considering any contributions.
+
+At this time, this guide is intended to be a living document. It still lacks
+important points and details, and future versions of the language and compiler
+may render some recommendations obsolete. We are eager to discuss and to receive
+contributions from anyone who is aware of common or uncommon pitfalls to avoid,
+or good coding practices and tools that can help building more robust software
+with the Rust language.
+
+Thus, feel free to create pull requests to suggest recommendations or
+modifications, or to submit an issue to start discussions. Specifically, please
+opt for a *pull request* for small changes like:
+
+- complementing a paragraph,
+- adding a small example in the form of code snippet,
+- updating some information
+- fixing typos and English mistakes,
+- etc.
+
+and for an *issue* in case of more substantive changes:
+
+- suggesting a new recommendation,
+- discussing controversial points,
+- rewording a consistent part of the text,
+- etc.