This line is meant to pertain to derivedderivative works which are maintained and shipped separately from the master. In such a case, you would need to keep these records (which is done automatically by source control)
What you have done however is not a derivedderivative work. You have committed your changes and have been pulled back to the main branch. Your changes are now part of the original project.
Furthermore, using source control (a public repository) means you will always be complying with this requirement.
There is the issue of how each person may define "prominent". To developers, source control(/+issue tracker) is a prominent way of viewing changes, however if you are maintaining a derivative work, you may want to maintain a list of substantial changes in a non-technical format.