Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contribution guide
+## Code style
+There are a lot of different code styles and developers' habits about how to write the code. 
+To resolve all differences we should follow one single code style guide:
+
+* all library sources and tests are written with [ES2015 and compiled by Babel.js](https://babeljs.io/docs/learn-es2015/)
+* We use [JSCS](http://jscs.info/) to check all sources according to preset rules of our code style.
+Please, check [JSCS configuration](./.jscsrc) and read the [docs](http://jscs.info/rules) about all used rules for code style guide.
+
+## Code linting
+We're using [ESLint](http://eslint.org/) as a static JS code linter. 
+Please, review our [ESLint configuration](./.eslintrc) and read the [ESLint docs](http://eslint.org/docs/rules/) about each rule.