Add Codecov badge

[alexmerlin]

[![codecov](https://codecov.io/gh/dotkernel/dot-helpers/graph/badge.svg?token=LIN5FVL5QP)](https://codecov.io/gh/dotkernel/dot-helpers)

Also:
README.md: please replace on the line with PHP from Packagist the string 3.4.0 with 3.4.1