docs: update contribution guide for new v4 build

CONTRIBUTING.md

@@ -1,33 +1,36 @@
 # Contributing to Cal-Heatmap
 
-## Editing source file
+## About the code
 
-All edition should be done on `src/cal-heatmap.js`.
+* The source code is located in the `src` folder
+* All tests are locacted in the  `test` folder
+* Code is written in Typescript
 
-**DO NOT** edit javascripts file located in the root, as they're built via grunt.
+## Getting started
 
-## Building final javascripts
+Clone/Download the repository, then run `npm install` 
+to insure you have all the required dev dependencies.
+
+## Code formatting
 
-Cal-Heatmap uses `grunt` to build final javascripts and stylesheets.
+* Code is formatted with ESLint/Prettier. See `.eslintrc.json` and `prettierrc`
 
-Install grunt via
+You can also trigger:
+* the lint with `npm run lint`
+* an auto-formatting with `npm run format`
 
-    npm install -g grunt-cli
+Ensure your code is formatted correctly before opening a pull request
 
-then run
+## Building bundle
 
-    grunt build
+As a developer, you should use `npm run dev` while developing, 
+as it produces a slimer and faster bundle.
 
-## Tests
-
-Cal-Heatmap is tested with [Qunit](https://qunitjs.com/).
-
-Tests files are located in the `test` directory, and can be run by opening the `index.html` file.
+The code is built with Rollup, with `npm run build`. This command is run 
+from the CI when releasing a new tag, as the bundle are not versioned.
 
-If you want to add or edit existing tests, **do not** edit the `test/test.js` file. Instead, add them in `test/src`, under the corresponding module. Take a look at existing tests for reference.
-
-You can then run `grunt build` to generate the final `test/test.js` file.
+## Tests
 
-## Submitting changes
+Cal-Heatmap is tested with Jest.
 
-Submit your pull requests with only files located in `src` folders. Do not commit files generated by grunt (.js located in the root and `test/test.js`).
+Tests are run with `npm run test`