Add marble diagram to the Single.never method

[UMFsimke]

First of all, I'm extremely grateful to open my first PR for RxJava. I hope that I'll be able to tick more of the marbles from #5788 but for now this should tick off the never operator.

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.

[codecov]

Codecov Report

Merging #6074 into 2.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6074      +/-   ##
============================================
+ Coverage     98.24%   98.28%   +0.03%     
+ Complexity     6192     6190       -2     
============================================
  Files           666      666              
  Lines         44806    44806              
  Branches       6206     6206              
============================================
+ Hits          44020    44036      +16     
+ Misses          239      236       -3     
+ Partials        547      534      -13

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	146 <0> (ø)	⬇️
...a/io/reactivex/internal/util/QueueDrainHelper.java	95.83% <0%> (-2.78%)	55% <0%> (-2%)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-2.14%)	5% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.2% <0%> (-1.45%)	2% <0%> (ø)
.../operators/mixed/FlowableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	98.29% <0%> (-0.86%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-0.66%)	2% <0%> (ø)
...ain/java/io/reactivex/subjects/UnicastSubject.java	99.36% <0%> (-0.64%)	64% <0%> (-1%)
... and 21 more

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[akarnokd]

https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.never.png

[UMFsimke]

Thank you! PR is updated :)