Add Quick start usage examples to README

[marius-bughiu]

Summary

• Adds a Quick start section to README.md with concrete, runnable usage examples for the common shapes: IntDictionary, CelerityDictionary (with GuidHasher, StringFnV1AHasher, and DefaultHasher<T>), the sets (IntSet, CeleritySet), and the IEnumerable<KeyValuePair<,>> constructor.
• Each snippet exercises the actual public API (indexer get/set, TryAdd / Add semantics, TryGetValue, Remove, bulk-load from a BCL Dictionary<,>).
• Closes the "Add usage examples to README" sub-task from the documentation backlog. The remaining sub-items on Improve documentation #15 (performance tuning guide, "when to use which collection", migration guide, troubleshooting / FAQ) remain open and will land in follow-up PRs.

Refs #15.

Test plan

• dotnet build — green (existing build, no source changes).
• dotnet test on CI — docs-only change; verifies nothing in the repo regressed.
• Visual review of the README — Quick start sits between Collections and Benchmarks; existing "Custom hashing" section is unchanged and cross-linked from Quick start.
• CHANGELOG [Unreleased] updated with the corresponding Added entry.

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