Add guide for switch to dev branch in CONTRIBUTING.md

CONTRIBUTING.md

@@ -21,6 +21,11 @@ an issue before starting writing your code. This helps us and the
 community to discuss the issue and choose what is deemed to be the
 best solution.
 
+## Switch to `dev` branch
+
+You should make your changes upon the `dev` branch (All development changes happen in the `dev` branch, then I publish a new version and merge them into the `master`)
+
+
 ## Drawing architecture
 We have a *_chart_painter.dart class per each chart type. It draws elements into the Canvas.
 We made the CanvasWrapper class, because we wanted to test draw functions.
@@ -49,6 +54,8 @@ Congratulations! Your code meets all of our guidelines :100:. Now you have to
 submit a pull request (or PR for short) to us. These are the steps you should
 follow when creating a PR:
 
+- Make sure you select `dev` branch as your target branch.
+
 - Make a descriptive title that summarizes what changes were in the PR.
 
 - Link to issues that this PR will fix (if any).