Add a README for /samples (#202)

samples/README.md

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+# Gemini API Swift SDK sample code
+
+This directory contains sample code for key features of the SDK, organised by high level feature.
+
+These samples are embedded in parts of the [documentation](https://ai.google.dev), most notably in the [API reference](https://ai.google.dev/api).
+
+Each file is structured as a runnable test case, ensuring that samples are executable and functional. Each test demonstrates a single concept, and contains region tags that are used to demarcate the test scaffolding from the spotlight code. If you are contributing, code within region tags should follow sample code best practices - being clear, complete and concise.
+
+## Contents
+
+| File | Description |
+| ---- | ----------- |
+| [APIKey.swift](./APIKey.swift) | Setting up your API key |
+| [ChatSnippets.swift](./ChatSnippets.swift) | Multi-turn chat conversations |
+| [CodeExecution.swift](./CodeExecution.swift) | Executing code |
+| [ControlledGeneration.swift](./ControlledGeneration.swift) | Generating content with output constraints (e.g. JSON mode) |
+| [CountTokens.swift](./CountTokens.swift) | Counting input and output tokens |
+| [FunctionCalling.swift](./FunctionCalling.swift) | Using function calling |
+| [GenerationConfig.swift](./GenerationConfig.swift) | Setting model parameters |
+| [SafetySettings.swift](./SafetySettings.swift) | Setting and using safety controls |
+| [SystemInstructions.swift](./SystemInstructions.swift) | Setting system instructions |
+| [TextGeneration.swift](./TextGeneration.swift) | Generating text |