docs: add install instructions to contibution guide (#2621)

Author: mbtools

CONTRIBUTING.md

@@ -8,6 +8,7 @@ In general, we follow the "fork-and-pull" Git workflow.
 1. [Fork](https://docs.github.com/en/get-started/exploring-projects-on-github/contributing-to-a-project) the repository on GitHub
 2. Clone the project to your local machine
 3. Work on your fork
+    * Install the project using `npm install --legacy-peer-deps` (see [issue](https://github.com/validatorjs/validator.js/issues/2123))
     * Make your changes and additions
         - Most of your changes should be focused on src/ and test/ folders and/or [README.md](https://github.com/validatorjs/validator.js/blob/master/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**.