Add Streamable class to README base classes section

[akarnokd]

Include Streamable in Readme.md .

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.5 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/main/java/io/reactivex/rxjava4/core/StreamSink.java: 4.735
src/main/java/io/reactivex/rxjava4/core/Streamable.java: 4.653
src/main/java/io/reactivex/rxjava4/core/VirtualEmitter.java: 4.832
src/main/java/io/reactivex/rxjava4/disposables/CompositeDisposable.java: 4.282
src/main/java/io/reactivex/rxjava4/disposables/DisposableContainer.java: 4.623
src/main/java/io/reactivex/rxjava4/disposables/DisposableStreamerCancellation.java: 4.885
src/main/java/io/reactivex/rxjava4/disposables/StreamerCancellation.java: 4.648
src/main/java/io/reactivex/rxjava4/internal/disposables/ListCompositeDisposable.java: 4.101
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamSinkWithCancellation.java: 4.917
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableConcatIterable.java: 4.187
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFlatMap.java: 4.166
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableForEach.java: 4.147
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableTakeUntil.java: 4.590
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableToObservable.java: 4.477
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableZip.java: 4.233
src/main/java/io/reactivex/rxjava4/internal/virtual/FlowableVirtualCreateExecutor.java: 4.525
src/main/java/io/reactivex/rxjava4/internal/virtual/FlowableVirtualTransformExecutor.java: 4.153
src/test/java/io/reactivex/rxjava4/validators/CheckParamValidationTest.java: 4.844

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.70%. Comparing base (97b7d04) to head (f2bd7ec).

Additional details and impacted files

@@            Coverage Diff            @@
##                4.x    #8224   +/-   ##
=========================================
  Coverage     98.69%   98.70%           
- Complexity     7095     7099    +4     
=========================================
  Files           813      813           
  Lines         50017    50017           
  Branches       6764     6764           
=========================================
+ Hits          49366    49368    +2     
+ Misses          452      448    -4     
- Partials        199      201    +2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.