Add marble diagram for Single.repeatUntil operator

[UMFsimke]

Here is marble for repeatUntil operator in #5788

Please let me know if I need to change anything on the diagram itself. If not, please send me URL when you upload image within the project so I can change URL for the marble in the PR.

[akarnokd]

https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.repeatUntil.png

[codecov]

Codecov Report

Merging #6084 into 2.x will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x   #6084      +/-   ##
===========================================
- Coverage     98.25%   98.2%   -0.05%     
+ Complexity     6194    6193       -1     
===========================================
  Files           667     667              
  Lines         44856   44858       +2     
  Branches       6211    6211              
===========================================
- Hits          44073   44054      -19     
- Misses          242     247       +5     
- Partials        541     557      +16

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	146 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-4.58%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	95.96% <0%> (-3.14%)	59% <0%> (-1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	95.83% <0%> (-2.78%)	55% <0%> (-2%)
...ternal/operators/observable/ObservablePublish.java	96.46% <0%> (-2.66%)	11% <0%> (+1%)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...ex/internal/operators/flowable/FlowableCreate.java	95.48% <0%> (-1.94%)	6% <0%> (ø)
...io/reactivex/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-1.07%)	5% <0%> (ø)
... and 18 more

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[UMFsimke]

Thanks! Updated