improved contributing.md

CONTRIBUTING.md

@@ -13,3 +13,10 @@ For testing or development, you can use that example page.
 In the `src/` folder you can find the source code, which is written in ES6 and compiled with babel. To build it, run `yarn build`, alternatively you can run `yarn watch` to keep babel running and automatically compiling your changes to the source code.
 
 If you're interested in contributing, you can start with open issues or testing and reviewing code and documentation.
+
+## Code style / project preferences
+To have a unique code style across the whole library, even if it's a tiny one, the following common options must be used in any piece of code:
+- Indenting with 2 spaces
+- Avoid useless comments (use them only if strictly needed)
+- We use `yarn` instead of `npm`
+- We use semantic versioning for releases (see http://semver.org/)