Update 4.12 Release Notes for #552

[Tibor17]

@dsaff
We are missing release notes update for #552.
Q: Maybe we were too fast and you did not see my remark in #552, so that having
getRuntime(TimeUnit) can be used in tests e.g. to assert in performance tests:

`@Test public void performanceTest() {
linering code;

duration= rule.getRuntime(SECONDS);
assertEquals(60, duration, 10); assert the test takes between 50 - 70 seconds

}`

The method immediately returns the time spent within the test until the call has been made.

[dsaff]

@Tibor17, do you have time to go ahead and update the release notes in the wiki?

https://github.com/KentBeck/junit/wiki/4.12-release-notes

[Tibor17]

@dsaff
Yes, I will update the release-notes next days.

Btw the release-notes for 4.11 is saying Unreleased in title. That's incorrect.
https://github.com/Tibor17/junit/blob/master/doc/ReleaseNotes4.11.md