Add StackOverflow links

[akarnokd]

Add StackOverflow links to Readme. Also trigger CI for codecov.io integration check.

[codecov]

Codecov Report

Merging #5046 into 2.x will increase coverage by 0.11%.

@@             Coverage Diff              @@
##                2.x    #5046      +/-   ##
============================================
+ Coverage     95.67%   95.79%   +0.11%     
============================================
  Files           612      612              
  Lines         39576    39576              
  Branches       5553     5553              
============================================
+ Hits          37866    37912      +46     
+ Misses          736      711      -25     
+ Partials        974      953      -21

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <ø> (-4%)	0% <ø> (-2%)
...internal/operators/completable/CompletableAmb.java	94.44% <ø> (-3.71%)	0% <ø> (-11%)
...x/internal/operators/maybe/MaybeSwitchIfEmpty.java	97.22% <ø> (-2.78%)	0% <ø> (-2%)
...main/java/io/reactivex/subjects/SingleSubject.java	97.61% <ø> (-2.39%)	0% <ø> (-40%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.6% <ø> (-2.2%)	0% <ø> (-2%)
...ain/java/io/reactivex/subjects/PublishSubject.java	97.87% <ø> (-2.13%)	0% <ø> (-41%)
.../io/reactivex/disposables/CompositeDisposable.java	97.24% <ø> (-1.84%)	0% <ø> (-40%)
...ternal/operators/flowable/FlowableSampleTimed.java	95.58% <ø> (-1.48%)	0% <ø> (-3%)
...io/reactivex/subscribers/SerializedSubscriber.java	92.04% <ø> (-1.14%)	0% <ø> (-25%)
...tivex/internal/operators/flowable/FlowableZip.java	91% <ø> (-1%)	0% <ø> (-6%)
... and 28 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1ea1291...6b80d2d. Read the comment docs.