3.x: Add Reactive Programming on Android with RxJava to Additional Docs

[arriolac]

• Adding "Reactive Programming on Android with RxJava", a free e-book, to additional reading.

[codecov]

Codecov Report

Merging #6665 into 3.x will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6665      +/-   ##
============================================
- Coverage     98.12%   98.06%   -0.06%     
- Complexity     6188     6189       +1     
============================================
  Files           677      677              
  Lines         44674    44674              
  Branches       6166     6166              
============================================
- Hits          43835    43811      -24     
- Misses          296      309      +13     
- Partials        543      554      +11

Impacted Files	Coverage Δ	Complexity Δ
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	90.47% <0%> (-3.97%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-3.21%)	5% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.14% <0%> (-2.61%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.44% <0%> (-1.17%)	2% <0%> (ø)
...3/internal/operators/flowable/FlowablePublish.java	97.53% <0%> (-0.99%)	16% <0%> (-1%)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.09% <0%> (-0.91%)	2% <0%> (ø)
...internal/operators/flowable/FlowableObserveOn.java	96.65% <0%> (-0.84%)	3% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-0.84%)	2% <0%> (ø)
... and 15 more

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