Add CONTRIBUTING.md

Author: felicio

CONTRIBUTING.md

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+# Contributing to ESLint-plugin-React
+
+## ESLint Rules
+
+### Markup
+
+Files documenting individual rules live in docs/rules folder. They're annotated
+using Markdown, which adds extra layer of legibility and better visual orientation
+to the documents. Consider this when writing supporting text for the rules.
+
+See the list below for some guidelines that need to be followed when adding to the
+collection.
+
+- Negative adverbs preceding snippets in 'Rule Details' sections must be **bold**.