added CONTRIBUTING.md file for better community contribution #170

CONTRIBUTING.md

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+The authors of `faker.js` has put lots of effort to create a useful and handy
+library. There is still a lot of things to be done, so all contributions are
+welcome! If you can make `faker.js` better, please read the following guide.
+
+# Important
+
+* Please make sure that you run both `gulp` and tests before making a PR.
+
+## Support
+
+`faker.js` relies on [commonJS](http://www.commonjs.org/) standard and supports both node.js and the
+browsers. Keep this in mind, when modifying and/or extending the sources.
+
+## Automation
+
+* The project is being built by [gulp](http://gulpjs.com/) (see [gulpfile](build/gulpfile.js)), destination directory is [build/build](build/build)
+* The documentation is auto-generated, basing on [build/src](build/src) markdown sources. If you modify the main [Readme.md](Readme.md) file, the PR will be rejected, since it will be overwritten by the upcoming `gulp` execution
+* The tests are executing `mocha` against all js contents of [test](test) directory
+
+## Architecture
+
+The sources are located in the [lib](lib) directory. All fake data generators are
+divided into namespaces (each namespace being a separate module). Most of the
+generators use the *definitions*, which are just plain JavaScript
+objects/arrays/strings that are separate for each [locale](lib/locales).