fix(README): update contribution section (#1143)

- close #1142

README.md

@@ -171,6 +171,8 @@ In general, we follow the "fork-and-pull" Git workflow.
 2. Clone the project to your own machine
 3. Work on your fork
     1. Make your changes and additions
+        - Most of your changes should be focused on `src/` and `test/` folders and/or `README.md`. 
+        - Files such as `validator.js`, `validator.min.js` and files in `lib/` folder are autogenerated when running tests (`npm test`) and need not to be changed **manually**. 
     2. Change or add tests if needed
     3. Run tests and make sure they pass
     4. Add changes to README.md if needed