Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+BRouter is heavily based on the following libraries:
+
+-   [Leaflet](leafletjs.com/) maps library, used in conjuction with many plugins.
+-   [Bootstrap](https://getbootstrap.com/) design library.
+-   [JQuery](https://jquery.com) javascript library.
+-   [Node.js](https://nodejs.org/) and [Yarn](https://yarnpkg.com/en/).
+
+## Install dependencies
+
+    yarn
+
+## Build
+
+```sh
+    #for development
+    yarn build debug
+
+    #for release
+    yarn build
+```
+
+## Develop
+
+    yarn serve
+
+## Translations
+
+`TL;DR` if you contribute to BRouter and add some translatable content, please make sure not to modify anything in `locales` folder, except `locales/en.json`. Full explanation below.
+
+### How internationalization works
+
+BRouter is translated using [i18next](https://www.i18next.com/) library, via command `gulp i18next`. It extracts translatable elements into `locales/en.json` file (English version).
+
+As soon as this file is modified, it must be uploaded to Transifex (manually) with the command `yarn push-transifex`.
+
+Anyone can then translate BRouter directly on [Transifex](https://www.transifex.com/openstreetmap/brouter-web/) platform.
+
+From time to time (eg. when preparing releases), we can update translated content with the command `yarn pull-transifex`. **It will overwrite all JSON files in `locales` directory**.
+
+## License
+
+BRouter is licensed under [MIT](LICENSE). Please make sure before adding any library that it is compatible with that. (GPL licenses are incompatible for instance).