2.x: Readme.md update snapshot links

[akarnokd]

Update the snapshot link to the upcoming 2.1.5+ and 2.2.x versions. Also I've added a convenience link to the actual directory listing of the various snapshots hosted by JFrog.

[codecov]

Codecov Report

Merging #5645 into 2.x will increase coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5645      +/-   ##
============================================
+ Coverage     96.18%   96.25%   +0.06%     
- Complexity     5846     5849       +3     
============================================
  Files           634      634              
  Lines         41539    41539              
  Branches       5752     5752              
============================================
+ Hits          39956    39985      +29     
+ Misses          628      613      -15     
+ Partials        955      941      -14

Impacted Files	Coverage Δ	Complexity Δ
...rnal/operators/observable/ObservableObserveOn.java	97.95% <0%> (-2.05%)	3% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	90.19% <0%> (-1.97%)	2% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	98.3% <0%> (-1.7%)	11% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
...java/io/reactivex/processors/UnicastProcessor.java	94.73% <0%> (-1.17%)	63% <0%> (-1%)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0%> (-1.13%)	6% <0%> (ø)
.../operators/observable/ObservableSequenceEqual.java	98.31% <0%> (-0.85%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
.../operators/observable/ObservableCombineLatest.java	96.31% <0%> (-0.62%)	6% <0%> (ø)
... and 21 more

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[artem-zinnatullin]

Just point to the second link directly instead of first one :)

[artem-zinnatullin]

I guess having 2 snapshots in this case kinda defeats purpose of snapshots,

Snapshot just always point to latest build, so I'd just update 2.0.0-DP0-SNAPSHOT to 2.2.0-SNAPSHOT (and maybe change snapshot artifact name to 2.x-SNAPSHOT in a separate PR