Document Services type for epics

[apples-kksk]

Summary

• explain what the Services type represents in redux-observable epic signatures
• show how to derive Services from an injected dependencies object
• add examples for wiring services into createEpicMiddleware and mocking them in tests

Fixes #103

Verification

• git diff --check

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #103: I would like an explanation of the Service type in epics.

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[gemini-code-assist]

Code Review

This pull request adds a new section to the README documentation explaining the Services type for redux-observable epics, including examples for service definition, middleware integration, and unit testing. A review comment identified a type mismatch in the documentation's example code where a string was passed to an action creator expecting an Error object, and a correction was suggested to maintain type safety.