Document usage of codecov.io service

[php-coder]

• link
• when it's running

[php-coder]

We don't use coveralls.io service anymore (see #755). I'm changing a title to take this fact into account: now we need to document usage of codecov.io service.

[php-coder]

We're going to stop using CodeCov at all. See #1105 for details.