docs: added info about target branch in new pull request

Author: piotr pietruszewski

CONTRIBUTING.md

@@ -104,6 +104,8 @@ The `package.json` file contains various scripts for common tasks:
 
 When you're sending a pull request:
 
+### Make sure that Pull Request target is `dev` branch.
+
 - Prefer small pull requests focused on one change.
 - Verify that linters and tests are passing.
 - Review the documentation to make sure it looks good.